Thanks Godfrey! This from thewikipedia page for "essence" is what I am remembering:
Ontological status
In his dialogues Plato suggests that concrete beings acquire their essence through their relations to "Forms"—abstract universals logically or ontologically separate from the objects of sense perception. These Forms are often put forth as the models or paradigms of which sensible things are "copies". When used in this sense, the word form is often capitalized.[5] Sensible bodies are in constant flux and imperfect and hence, by Plato's reckoning, less real than the Forms which are eternal, unchanging and complete. Typical examples of Forms given by Plato are largeness, smallness, equality, unity, goodness, beauty and justice.
Aristotle moves the Forms of Plato to the nucleus of the individual thing, which is called ousia or substance. Essence is the ti of the thing, the to ti en einai. Essence corresponds to the ousia's definition; essence is a real and physical aspect of the ousia (Aristotle, Metaphysics, I).