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).
• 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 to Use Definitions

• 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.'

Additional Tips for Writing Definitions

• 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."

  Better:

  "Rhyming poetry is an artform 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.