It may look simple, but writing a good definition is more than just writing description.
Each essay will differ depending on the topic, but they'll usually follow the same basic outline: As noted earlier, topics for these kinds of essays are usually an abstract idea or theory.
You could be given a topic such as 'Love' or 'Laziness' to define.
These are broad enough in scope that you can interpret them as you see fit, as long as you have the evidence to back it up.
It's a good idea to practice writing definition essays.