CHR-ROM
A cartridge's graphics memory — the pattern tables holding the actual pixels for tiles and sprites, kept apart from the code.
CHR-ROM sat on the PPU's side of the NES cartridge and mapped in at $0000–$1FFF, usually 8 KB at a time. It held the pattern tables: 8×8 cells of two bitplanes that the nametable and OAM pointed into. Because the graphics had their own chip, the PPU never fought the CPU for the same bus. Some cartridges used CHR-RAM instead and let the code rewrite tiles as the game ran — costlier but flexible. Larger mappers bank-switched CHR-ROM just like the program code, so a game could hold far more than the 512 tiles that fit at once.