Postumus (Marcus Cassianius Latinius Postumus) (pŏs`tyo͞oməs), d. 269?, Roman commander. Governor of Gaul under Gallienus, he revolted (257) and established an independent empire there. Although defeated (263) by Gallienus, Postumus escaped and continued to maintain his state until murdered by his own men. Postumus' measures weakened the authority of Rome, but in a sense he saved the West by creating a strong state that was able to check the barbarians.