Game file used by the game engine Z-machine, which was used for running text adventure games in the late 1970s and 80s; commonly run presently by a Z-machine interpreter, several of which have been maintained by community members since the Z-machine was discontinued; contains source code for the games.
Z-code files use a programming language called the Zork Implementation Language (Zork was Infocom's first game). The file extensions use a numerical value for the version of the Z-machine that is used to run them. Examples include the .Z1, .Z2, .Z4, .Z5, .Z6, .Z7, and .Z8 extensions. Since version 8 was the last Z-machine version, Z8 is the most popular of the extensions.
NOTE: Z3 files are much more common than Z1 and Z2 files but less common than later versions like the Z5 and Z8 extensions.