Phil: You never say "hizzy".
Joel: I do now! I'm a dynamic character!
People change; it's a fact of life if not nature. However, change can be gradual, and people may keep the core of their character intact for much if not all of their life, just as a traumatic experience may well abruptly change someone completely. Characters reflect that. When a character finishes a story with a different outlook or personality from when they started, they are called a Dynamic Character. It doesn't matter whether they had a Deep and Nuanced personality or started life as a cardboard cutout, or whether the character changes in either a subtle or overt way. We call this Character Development.
Mind you, this isn't always the case, nor is its absence bad. Contrast the Static Character who begins and ends much the same as they began a story. Maybe they've learned something, but it doesn't change how they act.
The change a dynamic character experiences can be born of just about anything. It might 'evolve' them from a flat to a round character, but it might as easily simply change them into another kind of flat or rounded character. Then again, it could well flatten a rounded character if they suffer some kind of Heroic BSoD or related trauma.