A small copper wire coil located in some hearing aids or cochlear implant. The telecoil, or T-coil, is used to enhance the signals from a telephone. They currently work with all land line phones and some cell phones. A telecoil may also be used in public places that are equipped with a loop system to improve speech understanding. It wirelessly connects the hearing aid or cochlear implant to the speaker’s microphone in places like theaters, places of worship, tour buses, classrooms, and auditoriums.
You can read more about telecoils here.