"Tremolo: A regular periodic change in amplitude, usually controlled by an LFO, with a periodicity of less than 20Hz. Compare with vibrato."
A lot of old guitar amplifiers, Fenders and others, had a built-in tremolo device; see "How Soon Is Now" by The Smiths for a real obvious example, and the deathless classic "Crimson and Clover" by Tommy James and the Shondells for a really fabulous example: They put tremolo on the voices, guitars, everything. "Cri-i-i-i-mso-o-on a-and cl-o-o-o-ve-er, o-o-o-ove-er a-a-and o-o-ve-er..." (and it's noded, too! Wharfinger jumps up and down at his desk, playing air guitar and singing, wildly off pitch -- "da-DA da-DA da-DAAAA", D major, A major, G major, over and over, over and over...).
Altering the tension on the strings of a guitar (hence pitch) by means of a vibrato tailpiece mechanism will, as a side effect, slightly alter the amplitude -- but that doesn't mean that vibrato and tremolo are the same thing. Ignorant semi-musicians love to use the word "tremolo" to refer to vibrato. Why? Because stupid people like to misuse words. Ignore them.