Chamber Pressure
The peak pressure generated inside the cartridge case at the instant the powder burns. Measured in PSI or C.U.P.
SAAMI publishes maximum average pressure (MAP) values for every commercial cartridge: 62,000 PSI for 6.5 Creedmoor, 65,000 PSI for .308 Winchester. Handloads should stay well under MAP, usually identified by working up in small charge increments and watching for pressure signs (flattened primers, sticky bolt lift, ejector marks on the case head).
Hot loads above MAP are dangerous: brass life shortens, primers can pierce, and in extreme cases the bolt or receiver can fail.