Flat

FLAT
A symbol placed in front of a note in a piece of music to lower it by one half step.

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Flat (Wikipedia)
In music, flat, or Bemolle, means "lower in pitch." More specifically, in music notation, flat means "lower in pitch by a semitone (half step)," and has an associated symbol (b), which is a stylised lowercase "b" . The Unicode character '♭' (U+266D) is the flat sign. Its HTML entity is ♭.
Under twelve tone equal temperament, C flat for instance is the same as, or enharmonically equivalent to, B natural, and G flat is the same as F sharp. Note that in any other tuning system, such enharmonic equivalences in general do not exist.
Double flats also exist, which look like and lower a note by two semitones, or a whole step. Less often (in for instance microtonal music notation) one will encounter half, or three-quarter, or otherwise altered flats. The Unicode character '' (U+1D12B) represents the double flat sign.
Although very uncommon and only used in modern classical music, a triple flat can sometimes be found. It lowers a note three semitones

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