PRG-ROM
A cartridge's program memory — the code and game data the CPU runs, kept apart from the graphics' own ROM.
An NES cartridge split its contents across two chips. PRG-ROM held the 6502 code and mapped into the CPU's address space at $8000–$FFFF, in windows of 16 or 32 KB. When a game outgrew the window, a mapper swapped in the right bank. The division — program here, graphics in CHR-ROM — let both be reached in parallel without sharing a bus, keeping the picture build fast. Sizes grew from 16 KB in the early titles to several hundred KB by the end of the console's life.