In terms of in-car entertainment, an FM transmitter is a small device you can plug in to your mp3 player. The transmitter then broadcasts your car stereo to an FM channel. This allows you to connect your music player to your car stereo without any rewiring.
An iPod accessory that streams FM radio from a car or home stereo to an iPod, and iPod contents to be streamed to a car or home stereo.
This article is concerned with low powered transmitters used in some countries for interfacing personal audio devices "FM transmitter" can also refer to high powered broadcast equipment used by pirate radio and licenced broadcast stations