Round-trip latency
The time from a note entering the audio interface to it coming back out of the headphones, after a lap through the computer.
The chain has several links: the A/D converter, the input buffer, processing in the DAW, the output buffer and the D/A converter. Buffers dominate. 128 samples at 48 kHz is 2.7 ms per buffer, and once you add a second buffer, the converters' filters and the driver's safety margin, a real measurement often lands around 6–8 ms — rarely the number printed on the box.
Around ten milliseconds is roughly where singers start hearing themselves as an echo. That is also about the time sound takes to cross three metres to a monitor, so a drummer in a large rehearsal room isn't necessarily better off. Anyone who needs zero latency listens to the input directly on the interface and keeps the computer out of the loop entirely.