Seed
Also said as Random seed, World seed, Pseudo-random number generator
The one number a random generator starts from. Computers fake randomness with maths, so the same seed always gives the same sequence, and so the same world.
A whole world can be shared as a few digits, or rebuilt rather than stored. It makes bugs reproducible, too. All of it breaks the moment anything else, such as the clock, sneaks into the sums.
Say it
“Generate the level from a seed, and print the seed on the death screen, so a player can send me the exact level the bug happened on.”
Where you’ve seen it
In Minecraft, anyone who enters the same seed in the same version gets the same world. No Man's Sky builds its 18 quintillion planets from a 64-bit seed, storing almost none of them.
Not to be confused with random
A seed does not make things more random. It makes randomness repeatable, and that is what you want it for.