Transponder

A transponder is a general term for a variety of devices that send and receive signals. Wireless transponders are used for a variety of applications including automated toll payment and access to gated communities; many cars have one or more attached to their windshield. Contactless smart cards are similar in concept (though usually more compact).

Links

 * Wikipedia article
 * Sunpass: one toll transponder system
 * E-Z Passes get read all over, not just at tolls