Action Replay

The Action Replay was a device for machines such as the Amiga and Commodore 64 which let you save the state of the machine to disk or tape, and recall it later. It meant you could save progress in games, to go back to if you lost too many lives. It was a mild form of cheat, something I did a lot with games like Dune 2 and Starcraft, where I would save many times through the course of a single map and, if things didn't work out, I'd reload a recent save and try again. It wasn't quite in the realm of infinite lives and invulnerability cheats that were common with 8-bit machines in the 80s, where patching a few bytes of game code stopped the game from decrementing your life count, or even activating the code that is meant to happen when you bump into a monster.

See the wikipedia page for more.