Some words make better definition essays than others. If your teacher allows you to choose your own word, term or phrase to write the essay on, choose one that is more abstract because abstract words lend themselves to more interesting essays. For example, we can easily define a "tree," "rock" or "house." On the other hand, words like "bravery," "patriotism," "love," "pleasure" and "pain" have different meanings based on an individual's interpretation.
You may find it helpful to choose a word or phrase that is personal to you. If you have first-hand experience with heroism, this word makes an apt choice. In the same vein, don't choose a word you have no experience with. If you have no first-hand experience with "monogamous dating," don't choose it.
Throughout history, words evolve. Many years ago, a word could have been insulting, but, as time passed, the meaning changed into something that may mean the exact opposite. Take a look at the way people use "bad" or "wicked." By definition, these words are clearly negative, but some people use the words in an opposite manner. Over time, these new definitions get included in dictionaries because, if people use them that way, that's what they mean. A history section can be an entire section of your essay, leading into how you personally define a word.
Many words have prefixes, suffixes and root words that make up their meanings. If you break the word apart, you'll make the definition clearer, and readers will have a stronger understanding of the word's meaning. This part goes well with the history, and, occasionally, it can introduce your history section.
People learn by relating something new to what they already know. If they don't already know your word, they may find it helpful if you list synonyms to help them get acquainted with your word. This can serve well as part of your introduction and can even open your essay if used effectively.