Source is a 3D game engine developed by Valve. It debuted as the successor to GoldSrc with Half-Life: Source in June 2004, followed shortly by Half-Life 2 in November, and has been in active development since. Source does not have a concise version numbering scheme; instead, it is designed in constant incremental updates. The engine began to be phased out by the late 2010s, with Source 2 succeeding it.
