Think about the terms you use and whether they need defining. If people in your subject area use the same term in different ways, then you have to set out at the beginning of your essay how you are going to use that term. That could be in a footnote, but it could also be in your text proper.
If you are using a lot of jargon, i.e. subject-specific terms a non-expert might not understand, it might be a good idea to include a glossary – a list of the terms and their explanations. You put the glossary either at the beginning of your essay (just before your title), or at the end of your essay. The first time you’re using a term in it tell your reader that you have a glossary, e.g. “a glorray of technical terms can be found on page ?”