A device for converting radio waves into an electrical signal, or visa versa. Common TV antennas are capable of transmitting as well as receiving. (TVs never transmit.)
If you feel you can learn about antennas more quickly from a "textbook" rather than a "reference book" then http://www.hdtvprimer.com/ISSUES/erecting_antenna.html is such a text.
If you feel you can learn about antennas more quickly from a "textbook" rather than a "reference book" then http://www.hdtvprimer.com/ISSUES/erecting_antenna.html is such a text.