The Collaborative International Dictionary of English v.0.48:

Peanut \Pea"nut\ (p[=e]"n[u^]t), n. (Bot.)
   The fruit of a trailing leguminous plant (Arachis
   hypog[ae]a); also, the plant itself, which is widely
   cultivated for its fruit.
   [1913 Webster]

   Note: The fruit is a hard pod, usually containing two or
         three seeds, sometimes but one, which ripen beneath the
         soil. Called also earthnut, groundnut, and
         goober.
         [1913 Webster]

WordNet (r) 3.0 (2006):

peanut
    adj 1: of little importance or influence or power; of minor
           status; "a minor, insignificant bureaucrat"; "peanut
           politicians" [syn: insignificant, peanut]
    n 1: underground pod of the peanut vine
    2: widely cultivated American plant cultivated in tropical and
       warm regions; showy yellow flowers on stalks that bend over
       to the soil so that seed pods ripen underground [syn:
       peanut, peanut vine, Arachis hypogaea]
    3: a young child who is small for his age
    4: pod of the peanut vine containing usually 2 nuts or seeds;
       `groundnut' and `monkey nut' are British terms [syn:
       peanut, earthnut, goober, goober pea, groundnut,
       monkey nut]