What is "zero-k"

Zero-K (ZK; formerly known as Complete Annihilation and CA) is a free multi-platform open source real-time strategy computer game. Initially based on content from Total Annihilation on the open source Spring Engine, it was forked and all proprietary content replaced, and evolved into a completely new game with unique features. Among the games powered by the Spring game engine, Zero-K is particularly notable for its extensive use of Lua scripting for interface and gameplay changes/enhancements, as well as unique RTS concepts such as a flat technology tree.