SuperGiantRobot: Why don't you just say "randomly generated"?
That is
literally what the term means. It just makes you sound stuck-up or like you'd found a thesaurus and want to use more expensive-sounding words to seem clever.
Geralt_of_Rivia: No, it doesn't.
It simply means that the game world/levels/whatever were not made by a human but were generated by the computer through an algorithm. That algorithm might include some form of randomization (and nowadays often does) but doesn't have to.
Actually, the very first game which used procedural generation (Elite from 1984) didn't use randomization at all, as far as I know.
A bit of correction. Firstly - a succinct definition is that the data is created by a computer / algorithm rather than manaually. and you are right, this can be done by either a random seed or a fixed seed.
The correction - Elite is not the first game that used procedural generation, as almost all games have been using some levels of procedural generation before then (and almost all games today does, just to save space). But you have games like Beneath Apple Manor from 1978,which came out 6 years before Elite, and used it a lot.
Anyway, the game I used to examplify procedural generation is Frontier: Elite 2. which is fixed and not random (in other words, every time you play you have the same galaxy). Frontier containes over 200,000,000,000 stars (most with orbiting planets and moons) and about 30,000 inhabited planet. It has space stations, a large variety of ships, a physics engine, seamless surface-to-space transitions and the actual gameplay. The closests stars to our sun are correct. On the Amiga this came on one singe floppy disk, and even then the disk was not full. If you look at some sites where you can downloaded it today, it is ~457 KB. The only reason why they managed to cram a whole galaxy into this size, is that the galaxy is procedurally generated.
This is also how it is used todya. take Skyrim - do you think that for example every single trree and leaf are made by hand? no, they are procedurally generated from a fixed template, just imagine the file size Skyrim would have needed if they where manually generated.
edit - and the same goes for modding. when you have a mod that changes the apperance of threes in Skyrim, do you think that the modder is sitting there and remoddeling eveyr single leaf in the game?