Caliber
The diameter of the bullet (and the bore that fires it), traditionally expressed in inches (.308) or millimeters (6.5mm).
Caliber is a measurement, not a cartridge name. A .308 bullet is .308 inches in diameter; that same bullet diameter is loaded into .308 Winchester, .300 Win Mag, .300 PRC, 30-06, and a dozen other cartridges.
When someone says “what caliber is that,” they usually mean what cartridge, which is colloquial usage. Both terms get used loosely; in precision work, name the cartridge explicitly so everyone stays on the same page.