What a thing is without which it would not be itself. Essence is a claim about necessity, and it is easy to overstate.
Latin essentia, a medieval coinage built on esse (to be) to answer a question Latin had no ready noun for: not that a thing is, but what its being consists of. The word passed into English through philosophy, keeping its abstract sense rather than settling into a plainer one.
The word itself had to be invented to ask a question Latin could not otherwise put into a noun: not that a thing is, but what its being is made of. Essence has stayed that abstract ever since, which is exactly why it is easy to reach for and hard to actually locate.