Prevent file from being changed under Linux

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 (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?

I have forgotten what this is called and without remembering the name the search function on the site and on Google is failing me.