contrast with existentialism; the idea that any thing that is a thing has the (predetermined) essence of that thing. What? Say, a chair has chairness. Its essence not only determines what it is, but what it will be and do. (Raising the question: is a broken chair still a chair?) The essence of water is wetness- you can't have dry water (even if it would be more portable, and easy to prepare). The essence of a woman (as a woman) is feminity. The essence is related to the ideal, in Platonic terms.
Disclaimer: just because i'm noding it doesn't mean i agree with it!