A Brief History
The original pre-Infocom game known either as Dungeon or Zork was written in MDL and could run on a mainframe computer. But the original Zork could not run on typical 80's home computers because of their memory and processor restrictions. Part of the solution, of course, was to split the game into three parts: Zork I, II and III. But more importantly, Infocom created ZIL by removing features of MDL that they didn't need, and by targeting ZIL for a virtual machine that they called the Z-machine.
Because the Z-machine was specifically designed for playing text adventures, the opcodes of its virtual processor could be likewise specifically designed. The opcodes of a Z-machine are called Z-code. The end result was that a game compiled from ZIL produced a story file small enough to fit on an 80's personal computer.
One more thing was needed: an interpreter to emulate the Z-machine. Infocom named their interpreter the Zork Interpreter Program ("ZIP"). A different version of ZIP was required for each computer model (Apple II, Atari, DEC Rainbow, etc.)
- e-ZIL, the Emacs Z-Machine Interface Language by Marc Simpson. (The e-ZIL project is now dead.)
- ZILF, a modern ZIL compiler.
- Z-machine, the virtual machine that ZIL and Inform compile to.