From The Collaborative International Dictionary of English v.0.48 :
vitamin B \vitamin B\ n.
a group of water-soluble vitamins originally thought to be a
single vitamin but now separated into several B vitamins.
Syn: B-complex vitamin, B complex, vitamin B complex, B
vitamin.
[WordNet 1.5]
From WordNet (r) 3.0 (2006) :
vitamin B
n 1: originally thought to be a single vitamin but now separated
into several B vitamins [syn: B-complex vitamin, B
complex, vitamin B complex, vitamin B, B vitamin,
B]