The Collaborative International Dictionary of English v.0.48:

Tremolo \Tre"mo*lo\, n. [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.
       [1913 Webster]