Full Idea
In 'Metaphysics' Z it seems that it is the form that provides the object with its identity.
Gist of Idea
Aristotle says that the form is what makes an entity what it is
Source
report of Aristotle (Metaphysics [c.324 BCE], Z) by Michael Frede - Aristotle's Conception of Metaphysics
Book Reference
Frede,Michael: 'Essays in Ancient Philosophy' [University of Minnesota 1987], p.89
A Reaction
See Aristotle on 'Hylomorphism' for what this means. By form he means a combination of structure, dispositions and controlling principles.