Serialisation
Also said as Saving state, Save data, Marshalling
Turning a live game's state into a flat stream of bytes that can go in a file or over a network, and back again. A save file is serialisation, and so is a network message.
Forget one field and the load looks fine while something is quietly wrong. Change the format later and everyone's old saves can break, unless you stored a version number from the start.
Say it
“Write a version number at the top of every save, and on load, upgrade old versions step by step. Add a test that saves and loads a full game, then compares every field.”
Where you’ve seen it
Minecraft stores its worlds in NBT, its own tree format for named binary data: level.dat for the world, one file per player, and chunks inside region files.
Not to be confused with save point
A save point is the design decision about when the player may save. Serialisation is the technical work of writing the game down at that moment.