Vibrato
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Vibrato is a musical effect where the pitch or frequency of a note or sound is quickly and repeatedly raised and lowered over a small distance for the duration of that note or sound. Sometimes, vibrato is erroneously referred to as tremolo, a periodic fluctuation in the amplitude (rather than frequency) of a sound.
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From Wikipedia discussion on the Vibrato article

Well my formal music training is classical (though my vocation is modern electronic music), and I've never heard vibrato refer to anything other than a rapid variation in pitch. Tremolo on the other hand, seems to be a bit of a catch-all word, sometimes meaning vibrato (especially in a derogatary way when talking about a singer doing it too much), sometimes rapidly repeating one note (especially on a mandolin or piano, with string instruments often having the word tremolando used instead), and sometimes rapidly changing volume (as in certain organs and groovy psychadelic guitar effects).
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