tremolo
Rapid repetition of a single note or alternation between two notes.
In Depth
Tremolo on a string instrument means rapidly repeating a single note by moving the bow back and forth as quickly as possible on the string. The result is a shimmering, sustained effect that can range from delicate and atmospheric to powerful and dramatic, depending on the dynamic level.
On a piano, tremolo involves rapid alternation between two notes or octaves. On mandolin and similar instruments, tremolo is the primary sustaining technique. In orchestral music, string tremolo is one of the most common effects — it appears in virtually every orchestral score and is particularly associated with suspense, drama, and atmospheric passages.
The tremolo effect was used so extensively in silent film scores that audiences began associating it with suspense and danger — an association that persists in film music to this day.