From WordNet (r) 3.0 (2006) [wn]:
grapefruit
n 1: citrus tree bearing large round edible fruit having a thick
yellow rind and juicy somewhat acid pulp [syn:
grapefruit, Citrus paradisi]
2: large yellow fruit with somewhat acid juicy pulp; usual
serving consists of a half
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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]