Definify.com
Webster 1913 Edition
Tremolo
‖
Tre′mo-lo
,Noun.
 [It. Cf. 
Tremulous
.] (Mus.) 
(a) 
The rapid reiteration of tones without any apparent cessation, so as to produce a tremulous effect. 
(b) 
A certain contrivance in an organ, which causes the notes to sound with rapid pulses or beats, producing a tremulous effect; – called also 
tremolant
, and tremulant
.