A bacteriophage (a virus which infects bacteria) that has single-stranded DNA. It is used as a method of obtaining single strands of foreign DNA so that the foreign DNA can be sequenced (that is, the order of its nucleotide bases can be determined). It is also used in procedures to create mutations in vitro (in a test tube rather than within an organism).

From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.