What symbol indicates to lower a note by a half step?

The symbol that indicates to lower a note by a half step is the flat sign, represented as ♭. When a note is preceded by this symbol, it instructs the musician to play that note at a pitch one semitone lower than its natural state. For example, if you have a B and you apply a flat to it (B♭), you would play B flat, which is the note just before B on a piano keyboard.

The sharp symbol (♯) does the opposite, raising a note by a half step. The natural sign (♮) cancels any previous sharps or flats and returns a note to its original, unaltered pitch. The symbol associated with diminished chords (∩) pertains to harmony and chord construction, but it does not signify a change in pitch for an individual note in the same way as the flat does. Therefore, the flat sign is specifically the correct answer when addressing the lowering of a note by a half step.