adult (from lat. adultus , part.parent.of adolescere , grow) adj. and n. It applies to the person or animal that has reached its full development or age in which it is suitable for procreation. adj It also applies, in a figurative sense, to non-organic things: An adult language . Fact. Adulthood, man, woman, adult age, adult. Acmé, climacteric, development, puberty; critical age.