Sample looping
Two markers inside a sample that make the sampler repeat a middle section for as long as the key is held.
A recorded flute note runs out after a few seconds, but a player may hold the key for a minute. The sampler handles this by playing the attack once, bouncing between the loop's start and end points while the note is held, then letting the rest of the sample play out as the release. When memory was measured in kilobytes, looping was also the only way to fit a piano in at all.
The hard part is the seam. Put the points mid-cycle and you get a click on every pass, so engineers hunt for zero crossings with matching slopes or blend the end into the start with a crossfade. Ping-pong loops, which play the section back and forth, hide the join but sound suspiciously static. The loop points live in the file format, for example in a WAV file's smpl chunk.