Definition is a rhetorical style that uses various techniques to impress upon the reader the meaning of a term, idea, or concept. Definition may be used for an entire essay but is often used as a rhetorical style within an essay that may mix rhetorical styles. For example, you may need to use definition in order to fully explain a concept before you make an argument about that concept.
A definition essay is structured around the goal of defining a term, concept, or idea. While you may start off with a simple dictionary definition, your essay will, ultimately, contain an extended definition. There are many techniques you can use to extend a definition in a definition essay (to be discussed further).