Headspace
The dimension that determines how far a cartridge case can move forward in the chamber before its head is supported by the bolt face.
A rifle in spec has its headspace measured at chambering and verified with go/no-go gauges. Excess headspace can cause case head separations, primer flow, or a ruptured case that vents gas. Insufficient headspace prevents the bolt from closing on a chambered round.
For handloaders, headspace also matters at the case level. Bumping the shoulder back during sizing too far creates excess headspace at the load level, shortening brass life and inflating ES.