Ok. Stupid question. I have forgotten what this is called and without remembering the name the search function on the site and on google is failing me.

What's the command under linux to mark a file as "locked"/to prevent any changes from being made to it? I'm not talking about chmod. There's a property that can be set (again the name escapes me at the moment) which prevents even processes running as root from changing a file. What is this called and how do I set it?