From OdaWiki

ACS (Action Coding Script?) is a compiled scripting language that works along side with the Hexen map format. The compiled scripts are stored in a map's BEHAVIOR lump in a binary format decoded by the zdoom/hexen engines. Odamex supports ACS inherited from ZDoom 1.22, however what works and what does not is not documented.

Specs and Resources - Unspecified author, found in the zdoom utilities source.