In music, tremolo is the quick repetition of one or two tones, usually for emotional effect. You might use a quavering tremolo when playing sad tunes on your violin.
A tremor is an unsteady, involuntary movement, like a shaky hand or a twitch. Similarly, tremolo is an unsteady, continuous repetition in music. Tremolo can involve one tone being quickly repeated or two tones being rapidly alternated. Tremolo often refers to a form of vibrato — when a singer holds and vibrates a tone — that is not controlled very well, and is considered showy and melodramatic. The root word is the Latin tremulus, or "trembling."