A bacteriophage (a virus which infects bacteria) that is useful to geneticists because it has a very strong promoter region which strongly encourages transcription of its gene by specific T7 RNA polymerase.
Geneticists can take the part with the promoter and attach their own genes of interest to it so that they can control transcription rates of their gene by choosing the amount of the RNA polymerase to put in.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.