Jitter (audio)
Tiny timing errors in the sample clock, so each sample is captured or played back slightly early or late.
Digital audio assumes samples sit at perfectly even intervals. When they don't, the converter measures the right value at the wrong moment, and the error turns into noise and sidebands around every tone. The higher the frequency, the faster the waveform moves and the more a given timing error costs: for a full-scale 20 kHz tone at 16 bits, around a hundred picoseconds is enough to disturb the least significant bit.
The classic culprits are S/PDIF and AES3, which embed the clock in the data stream and make the receiver rebuild it with a PLL that inherits every wobble from the sender and the cable. The cures are a shared word clock, reclocking at the receiver, or asynchronous USB, where the interface sets the pace and the computer merely keeps the buffer topped up. In hi-fi circles jitter has also become the explanation for almost anything, which makes it hard to discuss with a straight face.