Serverside Mods
The Half-Life serverside engine could originally only support one mod at a time, because it used to load a DLL which contained the standard engine as well as the mod.
Metamod is a serverside mod that fixed this, allowing multiple mods to be run at once. Many such mods run under Metamod, referred to as Metamod plugins.
The AMX Mod was a plugin running under Metamod that allowed scripts to be written for the servers. It got its name from the filename extension of compiled Pawn files, which stands for Abstract Machine eXecutive. In 2007 the AMX Mod project was merged into the related AMX Mod X, which had more features.
Read more about this topic: List Of Gold Src Engine Mods