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]

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]