The Collaborative International Dictionary of English v.0.48:

bacteriophage \bacteriophage\ n. sing. & pl.
   a virus which infects bacteria; -- also colloquially called
   phage in laboratory jargon.

   Note: Bacteriophages are of many varieties, generally
         specific for one or a narrow range of bacterial
         species, and almost every bacterium is susceptible to
         at least one bacteriophage. They may have DNA or RNA as
         their genetic component. Certain types of
         bacteriophage, called

   temperate bacteriophage, may infect but not kill their host
      bacteria, residing in and replicating either as a plasmid
      or integrated into the host genome. Under certain
      conditions, a resident temperate phage may become induced
      to multiply rapidly and vegetatively, killing and lysing
      its host bacterium, and producing multiple progeny. The
      lambda phage of Eschericia coli, much studied in
      biochemical and genetic research, is of the temperate
      type.
      [PJC] bacteriophagic

WordNet (r) 3.0 (2006):

bacteriophage
    n 1: a virus that is parasitic (reproduces itself) in bacteria;
         "phage uses the bacterium's machinery and energy to produce
         more phage until the bacterium is destroyed and phage is
         released to invade surrounding bacteria" [syn:
         bacteriophage, phage]