A musical term which can mean a good or a bad thing.

The good thing: When a note is flat, it is lowered a semitone or a half step from the note without the flat sign. A flat note is notated with a b-looking symbol preceding the note.

The bad thing: When a note is flat, its pitch is below the intended pitch. This is often caused by lack of singing or playing ability, fatigue, a badly tuned instrument, or lack of breath support.