PAK
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PAK file formats are package formats previously used by 
GoldSrc engine games to store content. It was first introduced in 
Quake engine which GoldSrc subsequently adopted it.
This was changed during the WON/Retail to Steam transition throughout 2003-2004, which the game files were stored inside GCF files located in 
...\Steam\steamapps\username\ folder, then later in 2013, with SteamPipe, the game file is now stored directly to the filesystem (example: ...\SteamApps\common\Half-Life\valve), as opposed to Source and Source 2 engines (which continued to packing game files inside multiple VPK files after SteamPipe transition) as most modern systems are fast enough to load GoldSrc game files quickly.
As of today, the Steam version of Half-Life, along with other GoldSrc games are still able to load the .pak file despite it's was no longer being used.
Important:PAK is often used as a generic file extension in the industry. PAK files used by other software, such as Chromium, Unreal Engine 4+, or CryEngine are completely unrelated to the file format used by GoldSrc and id Tech 2.