theme song

noun

1. a melody in an operetta or musical comedy so emphasized by repetition as to dominate the presentation.
2. a melody identifying or identified with a radio or television program, dance band, etc., usually played at the beginning of each program.
3. an expression, comment, or subject of conversation that a person or group uses habitually.

noun

1. a melody used, esp in a film score, to set a mood, introduce a character, etc
2. another term for signature tune