Flat, a musical character, shaped thus ♭, which, when placed before a note, lowers that note half a tone. The double flat, ♭♭, lowers a note two semitones. Singing or playing is called flat when the notes are at all below the right pitch.
Flat
Chambers's Encyclopaedia, Volume 4: Dionysius to Friction, p. 670
Source scan(s): p. 0687