Phage: Short for bacteriophage, a virus that lives within a bacteria. A virus for which the natural host is a bacterial cell.
Bacteriophages have been very important and heuristic in bacterial and molecular genetics. Phages were studied by (among others) Alfred Hershey, Max Delbruck and Salvador Luria who discovered that viruses could exchange genetic material. Hershey and a graduate student Martha Chase at the Cold Spring Harbor Laboratory found that nucleic acid alone could cause viral replication and transmit genetic information. The classic Hershey and Chase experiment provided some of the key proof that genes were made of DNA.
The "phage group" (Hershey, Delbruck and Luria) went on to share the 1969 Nobel Prize for Physiology or Medicine for their discoveries concerning "the replication mechanism and the genetic structure of viruses." Delbruck died in 1981, Luria in 1991, and Hershey in 1997. The famous "phage group" is thus gone but their legacy and that of the phages lives on.