Contra Lateral Routing of Signal (CROS) is a hearing aid technology for people with unilateral hearing. The technology allows two implementations: CROS and BiCROS.
Using this technology, a hearing aid-like device on the user's deaf side uses its microphone to pick up sound from that side and sends it to another instrument at the better ear. The sound is then inserted into the good ear.
The CROS implementation is for a user who has relatively normal hearing in the good side and has hearing that can't be aided on the bad side. The receiving BTE device on the bad side transmits the sound to a device on the good side. The user hears the amplified sound from the bad side in their good ear. The users hears the sound from the good side naturally in their good ear, without amplification.
The BiCROS implementation is for a user with little or no hearing on one side and with some hearing loss in their better ear. It works just like the CROS implemenation, except that the device on the good side is actually a fully capable hearing aid for hearing sounds from the good side that is also capable of receiving the sound transmitted from the CROS aid on the other side.
Transmission in a CROS or BiCROS configuration can be via wire around the back of the neck or wirelessly via radio.
The main advantages of CROS technology are: you can hear sounds coming from your deaf side better (even though you're hearing them in your better ear, and 2) you can get some cues about the location of the sound.
Primary disadvantages are: 1) the extra sound from the bad side may, at times, interfere with your ability to understand what you're hearing from your good side, and 2) CROS technology certainly can't restore natural localization and noise reduction, since that requires two ears, and CROS only inserts sound into one ear.
CROS technology may be a help for some people with unilateral hearing. BiCROS may help those whose good ear is also somewhat impaired. See the latest Google search for CROS and BiCROS.
People with unilateral hearing may also be interested in checking out the BAHA conductive hearing aid.
Finally, people with unilateral or noticeably unbalanced hearing loss may want to check out the information about Acoustic Neuromas.