An adult; a grown-up — someone who is not a child. Can also imply maturity or expected responsibility.
大人になると責任が増えるが、自由も増える。
When you become an adult responsibilities increase, but so does freedom.
この店は大人向けの落ち着いた雰囲気がある。
This shop has a calm atmosphere suited to adults.
子どもと大人では扱い方が違う場合がある。
There are cases where children and adults are handled differently.