Seed (level generation)

A seed is a code used in Minecraft's world generation process. Each world has its own seed value which is used to keep generated terrain consistent, as generation is pseudo-random. Prior to Beta 1.3, seeds were automatically generated per-world. Since then, the player can input their own seed, which is not limited to numbers due to the use of the Java function. For instance, a seed of "abc" is converted to the numeric value.

Contrary to common belief, typing in a biome name (i.e. tundra, desert, forest, etc.) as the seed does NOT always result in the creation of a world with primarily that biome. Nor is it guaranteed you will spawn in said biome when the map is generated.

Seeds determine more than just the area close to spawn. This is a common misconception as well. Seeds determine the entire landscape.

Using the same seed to generate two separate worlds will generate the same terrain consistently, even on different systems. As such, seeds can be shared by players to generate the same world between them. However, the player's initial spawn point will be random; it is not tied to the seed value. Longer seeds seem to be an exception to this rule, as they seem to give the same spawn point every time.

If you leave the seed blank, the game will use the time (computer clock) as a seed. A blank seed will not produce the same map every time.

The seed will only influence the numbers returned by the terrain generator, and won't influence its underlying behavior. For instance, there is no random seed that will give you a world with lava instead of water, or a purely desert world.