Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

citrus
     n 1: any of numerous fruits of the genus Citrus having thick rind
          and juicy pulp; grown in warm regions [syn: {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 tree}]