1.  grapefruit(n = noun.plant) citrus paradisi - citrus tree bearing large round edible fruit having a thick yellow rind and juicy somewhat acid pulp;
is a kind of citrus, citrus tree
has parts: grapefruit
is a member of genus citrus

2.  grapefruit(n = noun.food) - large yellow fruit with somewhat acid juicy pulp; usual serving consists of a half;
is a kind of citrous fruit, citrus, citrus fruit
is a part of citrus paradisi, grapefruit
has parts: grapefruit peel

1.  A citrus tree (Citrus paradisi) bearing large round edible fruit having a thick yellow rind and juicy somewhat acid pulp. [WordNet 1.5]

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

grapefruit, n. (pl. same)
1 a large round yellow citrus fruit with an acid juicy pulp.
2 the tree, Citrus paradisi, bearing this fruit.

grapefruit juice, grapefruit peel