Which notation indicates a note that is not part of the key signature?

The notation that indicates a note that is not part of the key signature is the accidental. Accidentals are symbols placed before a note to raise or lower its pitch and can include sharps, flats, and naturals. They temporarily alter the pitch of a note for the duration of that measure. This is crucial in music because it allows composers to introduce notes that are outside the established key signature, providing greater harmonic and melodic variety.

In contrast, the other options do not signify alterations to a note's pitch; therefore, they do not serve the same function as accidentals. For instance, repeat signs indicate sections of music that should be played again, dynamic markings relate to the volume or intensity of the music, and tempo indications inform the performer of the speed at which the piece should be played. These elements contribute to musical expression and structure but do not affect the pitch of individual notes within a composition.