Define a word like student, marriage, patriotism, or leader. The idea is not to look the word up in the dictionary, but for you to give the word a meaning you assign to it. Review essays in the chapter on definition and ask question in order to get a better idea how this is done.
The definition may be the point of the entire essay, meaning the thesis will state this is what marriage or patriotism or student or leader, for example, really means.
Or you can make another point in which understanding the word is essential to the point you are making.
Either way, you have to define a word the way view that word, a definition more about how you see it and not how it is defined in the dictionary. Abstract words like those above or love, courage, radicalism, or racism, leave room for more personal and debatable interpretations.