An Etymological Dictionary of Astronomy and Astrophysics

1. A person who is fully grown or developed or of age.
   

2. Having attained full size and strength; grown up; mature. A person who has attained the age of maturity as specified by law (Dictionary.com).

Etymology (EN): From L. adultus “grown up, mature, adult, ripe,” p.p. of adolescere “to grow up, mature,” from → ad- “to” + alescere “be nourished,” from alere “to nourish.”

Etymology (PE): Bornâ, from Mid.Pers. purnây- “adult;” Av. pərənāyu- “adult, old;” (Baluchi warnâ “adolescent”), literally “of full age,” from pərəna-, → full, + āyu- “age,” → aeon.