any mature animal
"The zookeeper separated the baby giraffes from their adult herd to ensure they had enough food."
A fully grown human or animal.
"The zookeeper checked on the newly hatched chick to see if it would survive until it became an adult."
In plain English: An adult is a grown-up person who has finished growing up and can take care of themselves.
"She is an adult who can make her own decisions."
Usage: Use "adult" as a noun to refer specifically to a fully grown person, such as in the phrase "the adults in the room." Avoid using it for young children who are simply older toddlers, as that usage contradicts its core meaning of being fully developed.
To behave like an adult.
"After years of blaming everyone else for their mistakes, Mark finally decided to act as an adult and take responsibility for his own life."
In plain English: To adult means to stop acting childish and start making responsible decisions like an adult would.
"The parents finally decided to adult and stop relying on their children for money."
Usage: Do not use "adult" as a verb to mean behaving maturely; this is a common error since the word functions only as a noun or adjective. Instead, choose synonyms like "act," "behave," or "conduct oneself" when describing someone showing maturity.
(of animals) fully developed
"an adult animal"
"a grown woman"
Fully grown.
"The caterpillar stopped eating and spun a cocoon, signaling that it was finally an adult butterfly ready to emerge."
In plain English: Adult means something that is fully grown and no longer a child.
"The adult men at the party were talking quietly in the corner."
Usage: Use "adult" as an adjective to describe something that has reached full growth or maturity, such as an adult plant or an adult film. Do not use it to refer to people who are merely old or elderly, since the term specifically implies having attained physical and mental development.
The word adult comes from the Latin phrase adultus, meaning "grown up." It entered English through French and is related to the word adolescent.