The Collaborative International Dictionary of English v.0.48:

Adult \A*dult"\, a. [L. adultus, p. p. of adolescere, akin to
   alere to nourish: cf. F. adulte. See Adolescent, Old.]
   Having arrived at maturity, or to full size and strength;
   matured; as, an adult person or plant; an adult ape; an adult
   age.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Adult \A*dult"\, n.
   A person, animal, or plant grown to full size and strength;
   one who has reached maturity.
   [1913 Webster]

   Note: In the common law, the term is applied to a person who
         has attained full age or legal majority; in the civil
         law, to males after the age of fourteen, and to females
         after twelve.
         [1913 Webster]

WordNet (r) 3.0 (2006):

adult
    adj 1: (of animals) fully developed; "an adult animal"; "a grown
           woman" [syn: adult, big, full-grown, fully grown,
           grown, grownup]
    2: designed to arouse lust; "pornographic films and magazines";
       "adult movies" [syn: pornographic, adult]
    n 1: a fully developed person from maturity onward [syn:
         adult, grownup] [ant: juvenile, juvenile person]
    2: any mature animal

Moby Thesaurus II by Grady Ward, 1.0:

19 Moby Thesaurus words for "adult":
   aged, big, full-blown, full-fledged, full-grown, grown, grown up,
   grown-up, marriable, marriageable, mature, matured, maturescent,
   nubile, of age, of marriageable age, old, ripe, ripened



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ADULT, in the civil law. An infant who, if a boy, has attained his full age
of fourteen years, and if a girl, her full age of twelve. Domat, Liv. Prel.
t. 2, s. 2, n. 8. In the common law an adult is considered one of full age.
1 Swanst. R. 553.