Full Idea
Matter is potentiality, whereas form is actuality.
Gist of Idea
Matter is potential, form is actual
Source
Aristotle (De Anima [c.329 BCE], 412a09)
A Reaction
Plato said mud has no Form. What did Aristotle think of that? I only ask because to me mud looks like unformed actuality.
Book Reference
Aristotle: 'De Anima (on the psuche)', ed/tr. Reeve, C.D.C. [Hackett 2017], p.21