An original MVS cartridge will contain ROM chips. ROM stands for Read-Only Memory. One of two basic types of memory. ROM contains only permanent information put there by the manufacturer; information in ROM cannot be altered, nor can the memory be dynamically allocated by the computer or its operator. The ROM chips in an original MVS game will be stamped by the game developer.
The nickname for a video game file on computer. ROMs are played on emulators. Unless it is a homebrew, it is illegal to possess a ROM under any circumstances (See Nintendo's excellent FAQ on the subject here). The usage of the term has little to do with the actual definition. See Emulator, homebrew, and ROM (1).