Full Idea
Change is the actuality of that which exists potentially, in so far as it is potentially this actuality. Thus, the actuality of a thing's capacity for alteration, in so far as it is a capacity for alteration, is alteration.
Gist of Idea
Change is the implied actuality of that which exists potentially
Source
Aristotle (Physics [c.337 BCE], 201a10)
Book Reference
Aristotle: 'Physics', ed/tr. Waterfield,Robin [OUP 1996], p.57
A Reaction
Not very informative, until you add Idea 16114, telling us that potentiality is best seen as 'power'. Then we have 'all change is the active expression of powers', which strikes me as rather interesting.
Related Idea
Idea 16114 Active 'dunamis' is best translated as 'power' or 'ability' (rather than 'potentiality') [Aristotle, by Gill,ML]