This is also known as the F factor or F-factor.
A small piece of episomal bacterial DNA which gives the bacteria that have it the ability to initiate conjugation (bacterial "mating") with other nearby bacteria. During conjugation, the F factor is copied and passed on to the other bacterium so that it acquires the ability to initiate conjugation also.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.