In stringed instruments, the quick reiteration of the same tone, produced by a rapid up-and-down movement of the bow.  The string tremolo is an important orchestral effect, widely used for dramatic passages or for the purpose of orchestral coloring.

In singing, "tremolo" commonly means the excessive vibrato that leads to deviation of pitch.  The true vocal tremolo is an effect that is practically never used today.  In early music, however, it was one of the most important ornaments.