What is "goldsrc"

GoldSrc (also known as Goldsource) was a game engine used internally by Valve Corporation. It was a heavily modified Quake engine that debuted with the release of their science fiction first-person shooter video game Half-Life. The engine also powered future titles developed by or with oversight from Valve, including Half-Life expansions, Day of Defeat, and multiple titles in the Counter-Strike series.

GoldSrc was eventually succeeded by the Source engine with the releases of Counter-Strike: Source and Half-Life 2 in 2004.