Full Idea
Definitions are thought to be of what something is, and what something is is in every case universal and positive.
Gist of Idea
Definitions are of what something is, and that is universal
Source
Aristotle (Posterior Analytics [c.327 BCE], 90b05)
Book Reference
Aristotle: 'Posterior Analytics (2nd ed)', ed/tr. Barnes,Jonathan [OUP 1993], p.49
A Reaction
This is exhibit A for those who think that Aristotelian essences concern the genus, rather than the particular. I suspect that this idea is best expressed as 'all we can say by way of definition of a particular thing involves the use of universals'.