The Collaborative International Dictionary of English v.0.48:

Cowpea \Cow"pea`\ (kou"p[=e]`), n.
   1. The seed of one or more leguminous plants of the genus
      Dolichos; also, the plant itself. Many varieties are
      cultivated in the southern part of the United States.
      [1913 Webster]

   2. (Bot.) A leguminous plant (Vigna Sinensis, syn. Vigna
      Catjang) found throughout the tropics of the Old World.
      It is extensively cultivated in the Southern United States
      for fodder, and the seed is used as food for man.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

cowpea
    n 1: fruit or seed of the cowpea plant [syn: cowpea, black-
         eyed pea]
    2: sprawling Old World annual cultivated especially in southern
       United States for food and forage and green manure [syn:
       cowpea, cowpea plant, black-eyed pea, Vigna
       unguiculata, Vigna sinensis]
    3: eaten fresh as shell beans or dried [syn: black-eyed pea,
       cowpea]