.FGD File Extension
File TypeForge Game Data File
What is an FGD file?
Game definition file used by Hammer Editor, a component of the Source SDK used to make map files for Valve games; describes all entities used by a game; enables game modifiers to reference game entities when creating custom game modifications.
FGD files are available for popular Valve games, such as Half-Life, Counter-Strike, and Team Fortress Classic.
NOTE: The Hammer Editor program was originally developed using the name "The Forge," which was used as the basis for naming FGD files.