Mudlib
The library of game code that runs on top of a MUD driver and supplies everything that makes the game a game: rooms, combat, commands, levels.
The LPMud family split the server into two layers. The driver — written in C, such as MudOS, FluffOS or DGD — only knows networking, memory and how to interpret LPC. The mudlib is everything else, written in LPC: what a room is, how a player logs in, what kill troll means. The same driver could therefore run a fantasy game, a science fiction setting or a pure chat hangout.
The early 2.4.5 mudlib was copied into hundreds of games. Later came polished alternatives such as Nightmare, TMI-2, Lima and the Discworld mudlib, often shared freely so others needn't start from scratch. DikuMUD has no such split — the game is hard-coded in C, which goes a long way to explaining why Diku games all feel so alike.