Several source ports have adopted support for using PK3 instead of, or in addition to, WAD files to distribute mods. However, contrarily to the PAK format used by Quake and Quake II, PK3 is not its own format, but merely a naming trick to prevent the operating system from associating these files with 'virtual folders' or archiving software, and to avoid leading users to believe they have to extract the content to make the game work. This nomenclature was introduced by Quake III Arena.
PK3 is an alternate extension for ZIP files.
