“Essay (French essai, “attempt”): a composition, usually in prose (Pope’s Moral Essays in verse are an exception), which may only be a few hundred words (like Bacon’s Essays) or book length (like Locke’s Essay Concerning Human Understanding) and which discusses, formally or informally, a topic or a variety of topics. It is one of the most flexible and adaptable of all literary forms….”
Excerpted from: Cuddon, J.A. The Penguin Dictionary of Literary Terms and Literary Theory. New York: Penguin, 1992.