A flat is a note that is lower in pitch by one half step (one semitone). The flat symbol (♭) indicates that a note is to be diminished (lowered) by one semitone.

 

Flat, in general, means, just lower in musical pitch. Singing and playing is called flat when the notes are below the correct pitch.

 

In a different sense, 'flat' describes a frequency response that is even at all frequencies, typically within 2 db.