The Collaborative International Dictionary of English v.0.48:

vitamin B complex \vitamin B complex\ n.
   a mixture of water-soluble vitamins originally thought to be
   a single vitamin but now known to be a mixture of several
   compounds with vitaminic activity.

   Syn: B complex, vitamin B complex, vitamin B, B vitamin.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

vitamin B complex
    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]