The martial keyword indicates that the power source of a class or power is martial arts training and physical prowess.

"Martial: Martial powers are not magic in the traditional sense, although some martial powers stand well beyond the capabilities of ordinary mortals." - PHB page 54.

Classes that draw power from the martial power source are:

Martial powers are also called exploits.

The other power sources are arcane, divine, primal, psionic, and shadow.