In music, flat (Italian bemolle for "soft B") means "lower in pitch". Flat is the opposite of sharp, which is a raising of pitch by the same amount (in Equal Temperament – by a near-equal amount in other temperaments). In musical notation, flat means "lower in pitch by one semitone (half step)". It is notated using the symbol ♭ preceding a note mark on the staff, or following a note written as a letter. The symbol is derived from a stylised lowercase 'b', possibly derived from German blatt ("planar").

When discussing intonation, flat can be used instead as an adjective, to mean "slightly lower in pitch" (by some unspecified amount). If two simultaneous notes are slightly out-of-tune, the lower-pitched one is "flat", assuming the higher one is properly pitched, and regardless of proper pitch, the lower-pitch is "flat" with respect to the slightly higher one. Furthermore, the verb flatten means to lower the pitch of a note, typically by an amount less than a semitone.