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):

Vigna sinensis
    n 1: 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]