1. The enlistment of people for the military, or for a job, or a cause etc.
2. In medicine, during the testing of hearing, the gradual increase to a maximum in a reflex when a stimulus of unaltered intensity is prolonged: In audiology, an abnormal recruitment of loudness caused by a very slight increase in sound strength of the acoustic stimulus in an ear with a sensory hearing loss compared with that of a normal ear.

From French, recrutement, "increase"; from Latin re-cresco , from -cretus, "to grow again" or recru, "new growth".