The Collaborative International Dictionary of English v.0.48:

Citrus \Cit"rus\ (s[i^]t"r[u^]s), n. [L., a citron tree.]
   1. (Bot.) A genus of trees including the orange, lemon, lime,
      tangerine, citron, grapefruit, etc., originally natives of
      southern Asia.
      [1913 Webster]

   2. (Bot.) any tree belonging to the genus Citrus, having
      leathery evergreen leaves and bearing a soft pulpy fruit
      covered by a thick skin; -- called also citrus tree.
      [PJC]

   3. (Bot.) the fruit of a tree belonging to the genus
      Citrus, having a thick shiny skin and a soft, sweet to
      tart pulp.
      [PJC]

WordNet (r) 3.0 (2006):

citrus
    n 1: any of numerous fruits of the genus Citrus having thick
         rind and juicy pulp; grown in warm regions [syn: citrus,
         citrus fruit, citrous fruit]
    2: any of numerous tropical usually thorny evergreen trees of
       the genus Citrus having leathery evergreen leaves and widely
       cultivated for their juicy edible fruits having leathery
       aromatic rinds [syn: citrus, citrus tree]