Full Idea
Soul is substance as the form of a natural body which potentially has life, and since this substance is actuality, soul will be the actuality of such a body.
Gist of Idea
Psuché is the form and actuality of a body which potentially has life
Source
Aristotle (De Anima [c.329 BCE], 412a20)
Book Reference
Aristotle: 'De Anima (On the Soul)', ed/tr. Lawson-Tancred,H.C. [Penguin 1986], p.157
A Reaction
To understand what Aristotle means by 'form' you must, I'm afraid, read the 'Metaphysics'. Form isn't shape, but rather the essence which bestows the individual identity on the thing. 'Psuche is the essence of man' might be a better slogan.