The Collaborative International Dictionary of English v.0.48:

Groundnut \Ground"nut`\ (ground"n[u^]t`), n. (Bot.)
   (a) The fruit of the Arachis hypog[ae]a (native country
       uncertain); the peanut; the earthnut.
   (b) A leguminous, twining plant (Apios tuberosa), producing
       clusters of dark purple flowers and having a root
       tuberous and pleasant to the taste.
   (c) The dwarf ginseng (Aralia trifolia). [U. S.] --Gray.
   (d) A European plant of the genus Bunium (B. flexuosum),
       having an edible root of a globular shape and sweet,
       aromatic taste; -- called also earthnut, earth
       chestnut, hawknut, and pignut.
       [1913 Webster]

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]

The Collaborative International Dictionary of English v.0.48:

Earthnut \Earth"nut`\, n. (Bot.)
   A name given to various roots, tubers, or pods grown under or
   on the ground; as to:
   (a) The esculent tubers of the umbelliferous plants Bunium
       flexuosum and Carum Bulbocastanum.
   (b) The peanut. See Peanut.
       [1913 Webster]

WordNet (r) 3.0 (2006):

earthnut
    n 1: any of various highly prized edible subterranean fungi of
         the genus Tuber; grow naturally in southwestern Europe
         [syn: truffle, earthnut, earth-ball]
    2: a common European plant having edible tubers with the flavor
       of roasted chestnuts [syn: earthnut, Conopodium
       denudatum]
    3: 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]
    4: edible subterranean fungus of the genus Tuber [syn:
       truffle, earthnut]