A lysogenic bacterium is one which contains in its genome the DNA of a virus which is lying dormant, passively letting itself be replicated by the bacterium whenever the bacterium replicates its own genome (a lysogenic virus). However, the virus is able to reactivate and destroy the bacterium at any time (it becomes a lytic virus).

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