Full Idea
Being one in form is just another way of saying one 'in definition'.
Gist of Idea
Unity of the form is just unity of the definition
Source
Aristotle (Physics [c.337 BCE], 190a16)
Book Reference
Aristotle: 'Physics', ed/tr. Waterfield,Robin [OUP 1996], p.25
A Reaction
I take this to be highly significant in understanding Aristotle. The crucial notion of form is tied to the way in which we understand the world, and does not refer to some independent fact about how it might really be.