a melody in an operetta or musical comedy so emphasized by repetition as to dominate the presentation.
a melody identifying or identified with a radio or television program, dance band, etc., usually played at the beginning of each program.
an expression, comment, or subject of conversation that a person or group uses habitually.
Origin of theme song
An Americanism dating back to 1925–30
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for theme song
a melody used, esp in a film score, to set a mood, introduce a character, etc
another term for signature tune
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012