I think "probability updates under Bayes' rule" is very clever and highly accurate, and it gets to just what you're talking about. Also, since this thread is trending towards everyone defining (or at least characterizing) rationality for themselves, here goes: rationality is what happens when evidence is recognized by a consciousness, subjected to ordered thought, and used to form or modify beliefs.

That's as close as I can get to "correct" for myself with a few minutes of thought and natural language. It seems to fit with the notion of rationality as a computational process.