Source is an extremely flexible/powerful video game engine developed by Valve Software, and it's primary use is for Half-Life 2, Counter-Strike: Source, and many Half-Life mods. But other games such as Vampires - Masqeurade: Bloodlines uses the same engine. It took Valve over 5 years of solid programming to develop Source.
Half-Life 2 runs on Source.
by Rebel Racer November 10, 2004