(1) The condition of a note which is, either deliberately or accidentally, lowered in pitch. This might be only a few cents or as much as a tone (double flat, bb). In music notation, a flat is indicated by the flat symbol (b), meaning lower the tone by a half-step.
(2) The same as dry. Unequalized, uncompressed, and otherwise unprocessed, describing a signal from any source: mic, instrument, or tape playback.
(3) The neutral position on a tone control, effecting no change to the signal. See center detent.
(4) Film projection using non-anamorphic lenses. In the U.S. the term flat is synonymous with an aspect ratio of 1.85:1 widescreen. See anamorphic.