The Collaborative International Dictionary of English v.0.48:

Citron \Cit"ron\ (s[i^]t"r[u^]n), n. [F. citron, LL. citro, fr.
   L. citrus citron tree (cf. citreum, sc. malum, a citron),
   from Gr. ki`tron citron]
   1. (Bot) A fruit resembling a lemon, but larger, and
      pleasantly aromatic; it is produced by the citron tree
      (Citrus medica). The thick rind, when candied, is the
      citron of commerce. The fruit was once called the lime.
      [1913 Webster]

   2. A citron tree, Citrus medica.
      [1913 Webster]

   3. A citron melon.
      [1913 Webster]

   Citron melon.
      (a) A small variety of muskmelon with sugary greenish
          flesh.
      (b) A small variety of watermelon, whose solid white flesh
          is used in making sweetmeats and preserves.

   Citron tree (Bot.), the tree which bears citrons. It was
      probably a native of northern India, and is now understood
      to be the typical form of Citrus Medica.
      [1913 Webster]

WordNet (r) 3.0 (2006):

citron tree
    n 1: thorny evergreen small tree or shrub of India widely
         cultivated for its large lemonlike fruits that have thick
         warty rind [syn: citron, citron tree, Citrus medica]