Xrun
Catch-all for audio buffer underruns and overruns: the software failed to fill or drain a buffer in time, and you hear it as a click.
The audio interface plays one buffer while the software computes the next. If the next one isn't ready when the hardware needs it, that's an underrun — nothing to play, so you get a pop or a stutter. Recording is the mirror image: the buffer fills faster than the software reads it and samples are lost in an overrun. The Linux audio world, where JACK and ALSA count and report them, lumped both together as xruns.
The trade-off never goes away. Smaller buffers mean lower latency but less slack when a driver, a network card or a power-saving feature borrows the CPU for a moment. Anyone tracking live ends up picking a buffer size just above where the crackling starts.