Save game
Writing down exactly enough of the game world's state to rebuild it later — and not a byte more.
Before cartridges had memory, players got a password that encoded their progress; Metroid (1986) is the famous example. The North American NES release of The Legend of Zelda carried a battery instead, keeping a small SRAM alive. The principle hasn't changed: you don't dump all of memory, you serialise whatever differs from the level's starting state — which doors are open, what's in the inventory, where the player is standing.
The hard part comes afterwards. Every patch that adds a field must still load old saves, so formats get versioned and migrated just like a database. Saving anywhere requires the entire state to be serialisable, which is why many games settle for checkpoints. And there will always be players who save before every dice roll and reload when luck turns.