FM Transmitter - definition
An FM transmitter allows a phone to broadcast music stored in its memory on FM frequencies, so that it can be tuned into a nearby FM receiver such as a a car radio.
FM transmitters are not a common feature on mobile phones, but such modern models do exist. The mobiles transmit at low power so interference with regular FM radio stations is highly likely – especially in dense urban environments where most of the FM spectrum is already populated by live radio broadcasts.
Some embedded FM transmitters allow for transmitting the music details over RDS.