A prophage is a bacteriophage genome. It is part of the circular bacterial DNA chromosome, or it is a plasmid outside the circular chromosome..
This is a latent form of a phage: the viral genes are there in the bacterium but cause no disruption of the bacterial cell.
Pro means "before", so, prophage means the stage of a virus inserted into host DNA before being activated inside the host.
Prophages are important agents of horizontal gene transfer.