Adult
Part of speech: noun
Definition: A fully grown human or animal. A person who lives outside the parental home. (improper usage) A person whose parents are deceased.
Part of speech: adjective
Definition: Of a human or animal, fully grown. Intended for adults rather than children. Of a film, etc, pornographic.
Example sentence: I feel very lucky to have grown up having interaction with adults who were making change but who were far from perfect beings. That feeling of not being paralyzed by your incredible inadequacy as a human being, which I feel every day, is a part of the legacy that I've gotten from so many of the adult elders.
Grownup
Part of speech: adjective
Definition: Of, pertaining to, or suitable for adults.Adult; fully developed; mature.
Part of speech: noun
Definition: An adult.