Hear from both sides - a Contralateral Routing Of Signals (CROS) hearing instrument is used to treat unilateral (or one-sided) hearing loss. It takes sound from the ear with poorer hearing and transmits to the ear with better hearing. A CROS is not a hearing aid - it requires a compatible hearing aid on your better ear to receive the sound.
A CROS is ideal where you have significant loss in one ear and have normal hearing in the other. A BiCROS solution is ideal where you are deaf in one ear and have a hearing loss in the other ear.