A definition essay is expository writing; it explains a word, idea or concept through personal commentary and reflection. Usually it will be an abstract idea. Everyone agrees what a tree or a car is, but you seldom see agreement on the definition of "love" or "success" -- these abstract words are topics that definition essays attempt to clarify, through a series of simple steps.
First Step, Word and Context
In step one, you select your idea to define and decide on its context. It should be arguable and complex in its differing uses. The word "success," for example, means something quite different to a young athlete than it does to an elderly retiree or a child fresh from a board game. Decide what the idea means to you and how you personally analyze it, remembering that you need enough varied examples to create an essay: what success meant to you in school, how you measure it now and so on.
Second Step, Thesis Statement
The second step is creating a thesis statement for the essay; the thesis statement explains the word in the context you are using. You can define a word by its function -- what it does -- or by its structure -- how it is organized -- or by analysis. The last, where you break down the different ways the word is used or understood, is the preferable method. It should lead to your personal experiences of "success," all of which should explain the concept and help readers understand it as you do.
Step Three, Body Examples
The third step is to fill your essay with examples and anecdotes that make the term and its context clear to your readers; your essay draft should support your concept with concrete details and commentary. The pattern is simple: begin a body paragraph with a topic sentence such as "Success in school to me meant praise from professors." Follow this with a concrete detail -- an anecdote about a successful class -- and commentary, analysis of the anecdote. Make sure to transition from one idea to another smoothly: "Another kind of personal success is--"
Final Step, Finishing
As with all essays, you revise and check your work, making sure your thesis is clear, concepts are organized, examples are valid, transitions are smooth. Be sure the essay defines and explains your concept as a whole. Now, if you apply to a college and your admission prompt happens to be "What does success mean to you?" -- you are certainly ready to answer.