In music or music theory a thirteenth is the note thirteen scale degrees from the root of chord and also the interval between the root and the thirteenth.

A thirteenth is a compound sixth.

A thirteenth chord is a chord with a thirteenth, and it is an extended chord if it includes the ninth and the eleventh. Added tone chords with what might be called an added thirteenth are usually said to have an added sixth, sixth chords, and are also called suspended chords.