A modification in a thing; the actualization of some potency possessed by a thing. 

Substantial Change.— A change that destroys a substance. E.g., the death of a human being.  Such a change is classically termed “corruption.”  The eduction of a new substance is thus referred to as “generation.” 

Accidental Change.— A change in the accidents of a substance; a change that does not necessarily destroy a substance. E.g., a tree grows a meter in height.