Source : Webster's Revised Unabridged Dictionary (1913)

Lime \Lime\, n. [F. lime; of Persian origin. See {Lemon}.]
   (Bot.)
   A fruit allied to the lemon, but much smaller; also, the tree
   which bears it. There are two kinds; {Citrus Medica}, var.
   acida which is intensely sour, and the sweet lime ({C.
   Medica}, var. Limetta) which is only slightly sour.

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. The thick rind, when candied, is the
      citron of commerce.

   2. A citron tree.

   3. A citron melon.

   {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}.

Source : WordNet®

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