Recruitment refers to a condition related to some hearing loss.
Recruitment causes your perception of sound to be exaggerated. Even though there is only a small increase in the noise levels, sound may seem much louder and it can distort and cause discomfort. Someone with recruitment can have problems only with specific sounds and frequencies or may have problems with all sound in general.
The theory of recruitment is that as the hair cells in your cochlea become ineffective, they "recruit" their (still working) neighbor hair cells to "hear" the frequency the damaged hair cell was supposed to hear, in addition to the frequency the still working hair cell was supposed to hear. This increases the signal from the still working hair cells.
The sounds reaching our brains appear to be much louder that normal. This is because the recruited hair cells still function in their original critical bands and also in the adjacent one(s) they have been recruited into.
The net effect is that people who have recruitment along with their hearing loss will experience an increasingly narrow range between the softest sound they can hear (caused by the hearing loss) and the loudest sound they can comfortably tolerate (caused by the recruitment).
Not everyone with hearing loss also has recruitment. It's a condition of the hair cells and their nerve endings in the cochlea. So, people whose hearing loss comes from other sources (such as conductive losses or nerve losses not involving the cochlea may not experience recruitment.
Thanks to Allan Feldt for suggesting this term.