A Contralateral Routing Of Signals (CROS) hearing aid is a type of hearing aid that is used to treat unilateral hearing loss. It takes sound from the ear with poorer hearing and transmits to the ear with better hearing.
Most systems are now wireless and are either behind the ear or custom built in-the-ear systems. These wireless systems have replaced earlier wired units which were bulky and rather fussy. Few people found them beneficial, and by contrast the wireless are easier to use and to wear. There are also systems incorporated into eyeglasses. BAHA and transcranial CROS systems use the conductivity of the skull to transmit sound. If hearing loss exists in the better ear then a bicros system is recommended
References
CROS hearing aid Wikipedia(Text) CC BY-SA