A definition is a passage that is used to define the meaning of a word, phrase, or concept. When writing an essay, supplying a good and clear definition is very important because a sloppy or misleading definition can less powerful or confusing. One formal definition should have 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


Cited from http://owl.english.purdue.edu/owl/resource


When to Use Definitions
If the author want to write a good article and avoid using unnecessary words, he should be much clarified about when and what particular point is needed to make explanation.
1.If the writer's describing object or concept is quite unfamiliar to readers, supplying a definition is essential.
2. When the writer wants to argue about such abstract or general concepts as "friendship" or "love", as different people may hold different opinions about these concepts, he needs to define the meaning in the passage.
3. When the writer adopts some different or new ideas of already known objects or generally accepted concepts, definition is used in order to clarify the point and add interest.

Some Methods
The writer frequently defines especially in expository and argumentation essays. His defining can be a single word, a sentence or two, a paragraph or even a chapter; and, at times, to make his definition clear, the writer needs the space of an entire book, for a definition is not complete until the writer can be sure that his reader knows what he has meant to tell. There is no one "correct" way one can write a definition. The writer may use any method or combination of methods known or desirable so long as he has efficiently brought his reader to understand what something is or what a word means.
1. Using of Synonyms
the writer may supply a synonym--a word or words of the same or similar meaning as the word being defined. A synonym for microbe, for example, is germ. If you want to find synonyms for words you are using, look in a thesaurus--a reference book containing words and their synonyms, or in a dictionary. But just by using synonyms is usually not powerful enough to put it through. Writers will have to use more explanatory phrase for definition when one word fails to make the meaning clear.
2. Exemplification
exemplification is one basic expository pattern used to help presenting, especially when that idea is quite unfamiliar or abstract to readers. A good example can help readers creating the image in their mind so that they can quickly understand writer's intention. E.g. the writer want to explain the tectonic structure of the earth, he may take the egg as an example.
3. Comparision
to help further persuading the reader, writers make use of the technique of contrast to illustrate. The thing-to-be-defined is placed in contrast with something similar, something with which it may be confused. Like if the writer wants define socialism, he may put it compared with capitalism, so that the readers will understand better.
more methods on http://www.termpaperscorner.com/articles/writing_a_definition_essay.html





Additional Tips:
  • Avoid circular definitions. Do not circle back to the word or root with which the word to be defined begins. Mere repetition has nothing help to your definition.
  • The definition should agree with your speech.
  • Try to use simple words and common phrases as you can as possible. Sentences filled with complicated words won't let your essay looks more professional. On the contrary, these words are obstacles in reader's comprehension process and this will make them less interested. Your essay should be understandable and clear.
  • Keep the length of the definition part adequate and essay well-balanced.


more tips can be found on http://www.wikihow.com/Write-a-Dictionary-Definition




References


1. http://owl.english.purdue.edu/owl/resource
2.http://plato.stanford.edu/entries/definitions/
3. http://www2.warwick.ac.uk/fac/soc/philosophy/staff/longworth/definitions.pdf
4. Robinson, Richard (1954). //Definition//. Oxford: At The Clarendon Press. ISBN 9780198241607.