In music, flat, or Bemolle, means "lower in pitch." In music notation, flat means "lower in pitch by a semitone (half step)," and has the symbol (♭). The Unicode character '♭' (U+266D) is the flat sign. Its HTML entity is ♭.
Double flats also exist, which look like and lower a note by two semitones, or a whole step. Sometimes you will encounter half or three-quarter flats. The Unicode character '𝄫' (U+1D12B) represents the double flat sign.
A triple flat can be found, but it is very rare. It lowers a note three semitones.
In tuning, flat can also mean "slightly lower in pitch". If the same note is played on two instruments at the same time, the lower-pitched one is flat compared to the higher-pitched one.
A half flat, showing quarter tones, is sometimes shown with a flat with a slash () or a reversed flat sign. A three-quarter flat is represented by a half flat and a regular flat.