Full Idea
Something holds of an item in itself if it holds of it in what it is - e.g., line of triangles and point of lines (their essence comes from these items, which inhere in the account which says what they are).
Gist of Idea
The essence of a triangle comes from the line, mentioned in any account of triangles
Source
Aristotle (Posterior Analytics [c.327 BCE], 73a35)
Book Reference
Aristotle: 'Posterior Analytics (2nd ed)', ed/tr. Barnes,Jonathan [OUP 1993], p.7
A Reaction
A helpful illustration of how a definition gives us the essence of something. You could not define triangles without mentioning straight lines. The lines are necessary features, but they are essential for any explanation, and for proper understanding.