It appears there isn't a single official "Lifesteal Server Adapter" tool specifically named that, but the term likely refers to the mod or plugin you use to add the Lifesteal game mode to your Minecraft server.
Here is a full description of what your Lifesteal Server Adapter (Plugin/Mod) does:
The Lifesteal adapter is the essential component that introduces the high-stakes, ruthless Lifesteal SMP mechanic to your server.
