The Collaborative International Dictionary of English v.0.48:

Lime \Lime\, n. [F. lime; of Persian origin. See Lemon.]
   1. (Bot.) The fruit of the Citrus aurantifolia, allied to
      the lemon, but greener in color; also, the tree which
      bears it.

   Note: The term lime was formerly also applied to variants of
         the closely related citron, of which there are two
         varieties, Citrus Medica, var. acida which is
         intensely sour, and the

   sweet lime (Citrus Medica, var. Limetta) which is only
      slightly sour. See citron.
      [1913 Webster +PJC]

   2. The color of the lime[1], a yellowish-green.
      [PJC]

WordNet (r) 3.0 (2006):

Citrus aurantifolia
    n 1: any of various related trees bearing limes [syn: lime,
         lime tree, Citrus aurantifolia]