Tremolo is a music word for one note, or two notes, played over and over very fast so the sound shakes or trembles instead of sitting still. Violin players make it by moving the bow quickly back and forth on one string; singers make it with a fast wobble in the voice; on an organ or a guitar amplifier a switch does it for you. The effect sounds tense or excited, so films use it under a scary or waiting moment. It is a countable noun ('a long tremolo') and also works in front of other words ('a tremolo effect', 'the tremolo arm on a guitar'). Two cautions about how people really talk: guitarists call the little bar that bends the pitch a 'tremolo arm', but that bar actually changes pitch, which is properly called vibrato — a famous mix-up that the industry never fixed. And ordinary listeners often say 'tremolo' for any shaky voice, including an old person's unsteady singing.