I'm curious about why you asked the second question. It seems obvious that "the word" you're talking about is human rationality, that being the whole focus of this community. So why ask people what the word you're asking about is? Is there something more subtle going on here?
This has been discussed some, but I don't think it's been the sole subject of a top-level post. I want to find out other people's ideas rather than driving the discussion into my ideas, so I'm asking the question in a very general form, and holding off on my own answers: