Full Idea
By the form of a thing, such as a changing human being, Aristotle means that which explains why the matter of this particular thing constitutes the thing that it constitutes: a particular human being.
Gist of Idea
The 'form' of a thing explains why the matter constitutes that particular thing
Source
report of Aristotle (Physics [c.337 BCE]) by Vassilis Politis - Aristotle and the Metaphysics 2.4
Book Reference
Politis,Vasilis: 'Aristotle and the Metaphysics' [Routledge 2004], p.53
A Reaction
If Politis is right then clearly the so-called 'formal cause' is much better understood as the 'formal explanation'. The Greek word for cause/explanation is 'aitia'.