The Collaborative International Dictionary of English v.0.48:

grapefruit \grape"fruit`\ (gr[=a]p"fr[=oo]t`), n.
   1. A citrus tree (Citrus paradisi) bearing large round
      edible fruit having a thick yellow rind and juicy somewhat
      acid pulp. [WordNet sense 1]

   Syn: Citrus paradisi.
        [WordNet 1.5]

   2. The large yellow fruit of the Citrus paradisi, having
      somewhat acid juicy pulp. It is a popular breakfast food.
      [WordNet sense 2]
      [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

Citrus paradisi
    n 1: citrus tree bearing large round edible fruit having a thick
         yellow rind and juicy somewhat acid pulp [syn:
         grapefruit, Citrus paradisi]