theme song

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An often repeated song in a musical play that is identified with the work or one of its characters.
• noun A song that is identified with a performer, group, or radio or television program.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun music, broadcasting A song which accompanies a television or radio program, usually as an intro or outro.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a melody that recurs and comes to represent a musical play or movie
• noun a melody used to identify a performer or a dance band or radio/tv program