In video games using procedural world generation, the map seed is a (relatively) short number or text string which is used to procedurally create the game world ("map").
Games which use procedural generation and include support for setting the map seed include Ark: Survival Evolved, Minecraft, Factorio, SCP – Containment Breach, and the desktop version of Terraria.
For Minecraft especially, there are websites[1][2][non-primary source needed] and articles,[3][4] dedicated to sharing seeds which have been found to generate interesting maps.
The map seed only has meaning in the context of the algorithm used to generate the map (that algorithm is often,[5][6] based on Perlin noise).
Such changes are particularly obvious in Minecraft, where they are handled (or rather, not handled) by simply generating any newly explored chunks of an existing map using the new algorithm, leading to obvious and jarring discontinuities after upgrading.