Also called a "lac I operon". A gene which is a part of the lac operon. It codes for a repressor protein, which prevents the transcription of genes that code for the enzymes that bring lactose into the bacterial cell and use it as a carbon source. The repressor protein prevents the unnecessary production of these enzymes when no lactose is present.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.