Rim
The flange at the base of a cartridge case. Lets the extractor grab the spent case and pull it out of the chamber.
Rims come in four major styles. Rimmed cartridges (.22 LR, .303 British) have a flange wider than the case body. Rimless (.308, .30-06, 6.5 Creedmoor) have the same diameter as the body. The extractor grabs the extractor groove cut just above the head. Semi-rimmed sits between the two, with a flange that protrudes only slightly past the body. Belted (.300 Win Mag) has a thickened band ahead of the extractor groove.
Most modern precision rifle cartridges are rimless because rimless feeds smoothly from box magazines.