Full Idea
For Aristotle, a person who has 'episteme' grasps the cause of a given phenomenon, can make a universal judgment about it, can explain it, and can teach others about it.
Clarification
'Episteme' is usually translated as 'scientific knowledge'
Gist of Idea
'Episteme' means grasping causes, universal judgments, explanation, and teaching
Source
report of Aristotle (Posterior Analytics [c.327 BCE]) by Charlotte Witt - Substance and Essence in Aristotle 1.2
Book Reference
Witt,Charlotte: 'Substance and Essence in Aristotle' [Cornell 1994], p.23
A Reaction
This I take to be the context in which we should understand what Aristotle means by an 'essence' - it is the source of all of the above, so it both makes a thing what it is, and explains why it shares features with other such things.