The Collaborative International Dictionary of English v.0.48:

Beet \Beet\ (b[=e]t), n. [AS. bete, from L. beta.]
   1. (Bot.) A biennial plant of the genus Beta, which
      produces an edible root the first year and seed the second
      year.
      [1913 Webster]

   2. The root of plants of the genus Beta, different species
      and varieties of which are used for the table, for feeding
      stock, or in making sugar.
      [1913 Webster]

   Note: There are many varieties of the common beet (Beta
         vulgaris). The Old "white beet", cultivated for its
         edible leafstalks, is a distinct species (Beta
         Cicla).
         [1913 Webster] Beete

WordNet (r) 3.0 (2006):

beet
    n 1: biennial Eurasian plant usually having a swollen edible
         root; widely cultivated as a food crop [syn: beet,
         common beet, Beta vulgaris]
    2: round red root vegetable [syn: beet, beetroot]