A audio term, basically refering to slightly pitch modulating
a tone to give it a smoother sound. So, if I have a
tone at 440 Hz, I might modulate that using a 5 Hz
sine wave, so it swings between 435 and 445 Hz.
More advanced techniques increase the amount
of modulation over time, so intiially it may have no
vibrato, but after a second it might be 1% of the original
frequency, 2% after 2 seconds, and so on.