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3.1: What Is a Definition?

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    An important writing tool you'll need, particularly if you are writing for non-specialists, is the definition—or more specifically, extended definition. An extended definition is one or more paragraphs that attempt to explain a complex term. Some terms may be important in your report, there may be much confusion about them, or they may be difficult to understand that an extended discussion is vital for the success of your report.

    When you write reports, you may often discover that you need to explain certain basics before you can discuss the main subject matter. For example:

    • In a report on new treatments for sickle cell anemia, you'd need a section defining the disease.
    • In a report on the benefits of drip irrigation, you'd need to write an extended definition of drip irrigation, explaining how it works and what equipment is used.
    • In a report showing small businesses how to weather economic recessions, an extended definition of the term “economic recession” would be needed first.


    One of the first things to do when you write an extended definition is to compose the formal sentence definition of the term you are writing about. Place it toward the beginning of the extended definition. This formal definition establishes the focus for the rest of the discussion. It is "formal" because it uses a certain form. Here are several examples:

    Figure \(\PageIndex{1}\): Formal Sentence Definitions

    Formal sentence definitions: their components are the term being defined, the class it belongs to, and its distinguishing characteristics.

    Take particular care when you write the reference to the class to which the term belongs; it sets up a larger frame of reference or context. It gives readers something familiar to associate the term with. The term may belong to a class of tools, diseases, geological processes, electronic components; it may be a term from the field of medicine, computer science, agriculture, reprographics, or finance. Avoid vague references to the class the term belongs to; for example, instead of calling a concussion an "injury" or botulism a "medical problem," call them something more specific like "a serious head injury" and "a severe form of food poisoning," respectively.

    Similarly, provide plenty of specific detail in the characteristics component of the formal sentence definition. Readers need these details to begin forming their own understanding of the term you are defining.

    Be aware, however, that your formal sentence definition will likely contain additional potentially unfamiliar terms. Somewhere in your extended definition, you'll need to explain them as well, possibly by using short definitions (explained later in this section). A formal sentence definition used in an extended definition is below:

    Figure \(\PageIndex{2}\): Extended Definition I


    When you write an extended definition, you literally grab at any of the writing resources or tools that will help you explain the term to your readers. This means considering all of the various sources of information that can help define the term adequately (for example, description, process narration, causal discussion, and classification).

    The key to writing a good extended definition is to choose the sources of definition to help readers understand the term being defined. Use this checklist to select the kinds of discussion to include in your extended definitions:

    Figure \(\PageIndex{3}\): Sources of Extended Definitions

    Below is an outline of a report that uses an extended definition. This view shows how different sources of definition can be used to write an extended definition.

    Figure \(\PageIndex{4}\): Extended Definition Outline

    Below is another extended definition. This one uses additional definitions, descriptions (demographics), and process.

    Figure \(\PageIndex{5}\): Extended Definition II


    You'll find that in writing an extended definition, you must define other terms as well. Typically, short definitions—a sentence, clause, or phrase in length—will suffice. Notice how many are added to the "after" version in the following.

    Figure \(\PageIndex{6}\): Adding Short Definitions Before & After

    This process of supplying short definitions "on the fly" is critical in good technical writing for non-specialists. Notice how many quick definitions occur just in the first two sentences of the preceding illustration. "Maculopapular" is defined in parentheses as "(raised red)." "Endemicity" is defined by restating the idea in other words: "that is, people throughout the world are capable of contracting measles." And "infective particle" is quickly defined by providing an alternative: "or organism causing the illness." Obviously, the passage is almost tripled in length—but that's the price for thorough explanation and clarity.


    Extended definitions don't call out for any special format; just use headings, lists, notices, and graphics as you would in any other technical document.

    Below is a schematic view of an extended definition. Remember that this is just a typical or common model for the contents and organization—many others are possible.

    Figure \(\PageIndex{7}\): Format for Extended Definition


    Below is an example of an extended definition paper on Sickle Cell Anemia. Note: A technical writing students at the University of Texas at Austin wrote this definition in the 1980s. Technical accuracy cannot be guaranteed.


    Sickle cell anemia is congenital hemolytic anemia that occurs primarily but not exclusively in African-Americans. The condition results from a defective hemoglobin molecule (hemoglobin S) which causes red blood cells (RBCs) to roughen and become sickle-shaped. Such cells impair circulation, resulting in chronic ill-health (fatigue, dyspnea on exertion, and swollen joints), periodic crises, long-term complications, and premature death. At present, only symptomatic treatment is available. Half of such patients die by their early 20s; few live to middle age.


    Sickle cell anemia results from homozygous inheritance of the hemoglobin S-producing gene, which causes substitution of the amino acid valine for glutamic acid in the B hemoglobin chain. Heterozygous inheritance of this gene results in sickle cell trait, generally an asymptomatic condition. Sickle cell anemia is most common in tropical Africans and in persons of African descent. About 1 in 10 African-Americans carries the abnormal gene. If two such carriers have offspring, there is a 1 in 4 chance that each child will have the disease. Overall, 1 in every 400 to 600 African-American children has sickle cell anemia. This disease also occurs in Puerto Rico, Turkey, India, the Middle East, and the Mediterranean area. Possibly, the defective hemoglobin S-producing gene has persisted because, in areas where malaria is endemic, the heterozygous sickle cell trait provides resistance to malaria and is actually beneficial.

    Figure \(\PageIndex{8}\): Inheritance of Sickle Cell

    The abnormal hemoglobin S found in such patients' RBCs becomes insoluble whenever hypoxia occurs. As a result, these RBCs become rigid, rough, and elongated, forming a crescent or sickle shape. Such sickling can produce hemolysis (cell destruction). In addition, these altered cells tend to pile up in capillaries and smaller blood vessels, making the blood more viscous. Normal circulation is impaired, causing pain, tissue infarctions, and swelling. Such blockage causes anoxic changes that lead to further sickling and obstruction.

    Figure \(\PageIndex{9}\): Normal Red Blood Cell vs. Sickle Cells


    A number of symptoms are associated with sickle cell anemia and in particular several types of crises.

    Symptoms. Characteristically, sickle cell anemia produces tachycardia, cardiomegaly, systolic and diastolic murmurs, pulmonary infarctions (which may result in cor pulmonale), chronic fatigue, unexplained dyspnea or dyspnea on exertion, hepatomegaly, jaundice, pallor, joint swelling, aching bones, chest pains, ischemic leg ulcers (especially around the ankles), and increased susceptibility to infection. Such symptoms usually don't develop until after 6 months of age, since large amounts of fetal hemoglobin protect infants for the first few months after birth. Low socioeconomic status and related problems, such as poor nutrition and low educational levels, may delay diagnosis and supportive treatment.

    Periodic crises. Infection, stress, dehydration, and conditions that provoke hypoxia—strenuous exercise, high altitude, unpressurized aircraft, cold, and vasoconstrictive drugs—may all provoke periodic crisis. Painful crisis. A painful crisis (vaso-occlusive crisis, infarctive crisis), the most common crisis and the hallmark of this disease, usually doesn't appear until age 5 but recurs periodically thereafter. It results from blood vessel obstruction by rigid, tangled sickle cells, which causes tissue anoxia and possibly necrosis. It is characterized by severe abdominal, thoracic, muscular, or bone pain and possibly increased jaundice, dark urine, or a low-grade fever. Autosplenectomy, in which splenic damage and scarring is so extensive that the spleen shrinks and becomes impalpable, occurs in patients with long-term disease. Such autosplenectomy can lead to increased susceptibility to Diplococcus pneumoniae sepsis, which can be fatal without prompt treatment. After the symptoms of crisis subside (in 4 days or several weeks), infection may develop, often indicated by lethargy, sleepiness, fever, or apathy.

    Anaplastic crisis. Associated with infection is the anaplastic crisis (megaloblastic crisis) which results from bone marrow depression and is associated with infection, usually viral. It is characterized by pallor, lethargy, sleepiness, dyspnea, possible coma, markedly decreased bone marrow activity, and RBC hemolysis.

    Acute sequestration crisis. In infants between 8 months and 2 years old, an acute sequestration crisis may cause sudden massive entrapment of red cells in the spleen and liver. This rare crisis causes lethargy and pallor, and if untreated, can progress hypovolemic shock and death. In fact, it's the most common cause of death in sickle cell children under 1 year.

    Hemolytic crisis. Hemolytic crises are quite rare and usually occur in patients who have glucose-6-phosphate dehydrogenase (G-6-PD) deficiency with sickle cell anemia. It probably results from complications of sickle cell anemia, such as infection, rather than from the disorder itself. The hemolytic crisis causes liver congestion and hepatomegaly as a result of degenerative changes. It worsens chronic jaundice although increased jaundice doesn't always point to a hemolytic crisis.

    Any of these crises are possible in sickle cell anemia patients with pale lips, tongue, palms, or nail beds; lethargy; listlessness; sleepiness, with difficulty awakening; irritability; severe pain; temperature over 104° F (40° C) or a fever of 100# F (38° C) that persists for 2 days.

    Long-term complications. Sickle cell anemia also causes a number of long-term complications. Typically, children with sickle cell anemia are small for the age, and puberty is delayed. (However, fertility is not impaired.) If they reach adulthood, their bodies tend to be spiderlike—narrow shoulders and hips, long extremities, curved spine, barrel chest, and elongated skull. An adult usually has complications stemming from infarction of various organs, such as retinopathy and nephropathy. Premature death usually results from infection or repeated occlusion of small blood vessels and consequent infarction or necrosis of major organs. For example, cerebral blood vessel occlusion causes a cerebrovascular accident.


    A positive family history and typical clinical features suggest sickle cell anemia; a stained blood smear showing sickle cells, and hemoglobin electrophoresis showing hemoglobin S confirm it. Ideally, electrophoresis should be done on umbilical cord blood samples at birth, especially if the parents are known to carry the sickle cell trait. Additional lab studies show low RBC, elevated WBC and platelet count, decreased erythrocyte sedimentation rate (ESR), increased serum iron, decreased RBC survival, and reticulocytosis. Hemoglobin may be low or normal. During early childhood, palpation may reveal splenomegaly, but as the child grows older, the spleen shrinks and splenic function is impaired.


    Treatment is primarily symptomatic and can usually take place at home. If the patient's hemoglobin drops suddenly, as in an anaplastic crisis, or if his condition deteriorates rapidly, hospitalization is needed for transfusion of packed red cells. In a sequestration crisis, treatment may include blood transfusion, oxygen administration, and large amounts of oral or IV fluids. So far, research to find an effective antisickling agent hasn't been successful.


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    This handout provides suggestions and examples for writing definitions.

    Contributors: Mark Pepper, Dana Lynn Driscoll

    Last Edited: 2015-07-30 12:09:50

    A formal definition is based upon a concise, logical pattern that includes as much information as it can within a minimum amount of space. The primary reason to include definitions in your writing is to avoid misunderstanding with your audience. A formal definition consists of three parts.

    1. The term (word or phrase) to be defined
    2. The class of object or concept to which the term belongs.
    3. The differentiating characteristics that distinguish it from all others
      of its class

    For example:

    • Water (term) is a liquid (class) made up of molecules of hydrogen and oxygen in the ratio of 2 to 1 (differentiating characteristics).
    • Comic books (term) are sequential and narrative publications (class) consisting of illustrations, captions, dialogue balloons, and often focus on super-powered heroes (differentiating characteristics).con
    • Astronomy (term) is a branch of scientific study (class) primarily concerned with celestial objects inside and outside of the earth's atmosphere (differentiating characteristics).

    Although these examples should illustrate the manner in which the three parts work together, they are not the most realistic cases. Most readers will already be quite familiar with the concepts of water, comic books, and astronomy. For this reason, it is important to know when and why you should include definitions in your writing.


    • When your writing contains a term that may be key to audience understanding and that term could likely be unfamiliar to them
      • "Stellar Wobble is a measurable variation of speed wherein a star's velocity is shifted by the gravitational pull of a foreign body."
    • When a commonly used word or phrase has layers of subjectivity or evaluation in the way you choose to define it
      • "Throughout this essay, the term classic gaming will refer specifically to playing video games produced for the Atari, the original Nintendo Entertainment System, and any systems in-between."
      • Note: not everyone may define "classic gaming" within this same time span; therefore, it is important to define your terms
    • When the etymology (origin and history) of a common word might prove interesting or will help expand upon a point
      • "Pagan can be traced back to Roman military slang for an incompetent soldier. In this sense, Christians who consider themselves soldiers of Christ are using the term not only to suggest a person's secular status but also their lack of bravery.'


    • Avoid defining with "X is when" and "X is where" statements. These introductory adverb phrases should be avoided. Define a noun with a noun, a verb with a verb, and so forth.
    • Do not define a word by mere repetition or merely restating the word.
      "Rhyming poetry consists of lines that contain end rhymes."
      "Rhyming poetry is an art form consisting of lines whose final words consistently contain identical, final stressed vowel sounds."
    • Define a word in simple and familiar terms. Your definition of an unfamiliar word should not lead your audience towards looking up more words in order to understand your definition.
    • Keep the class portion of your definition small but adequate. It should be large enough to include all members of the term you are defining but no larger. Avoid adding personal details to definitions. Although you may think the story about your Grandfather will perfectly encapsulate the concept of stinginess, your audience may fail to relate. Offering personal definitions may only increase the likeliness of misinterpretation that you are trying to avoid.


    This page titled 3.1: What Is a Definition? is shared under a CC BY license and was authored, remixed, and/or curated by Chelsea Milbourne, Anne Regan, Morgan Livingston, & Sadie Johann.

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