Having a truth value at or near unity.
In contrast to faith certainty generally means confidence in one's operating assumptions, proportional to their verifiability and openness to rational criticism. We can describe certainty as the result of reason where faith carries no such or even opposite causal assumptions.
I'm not sure that the word carries any connotations of whether faith or reason was used to assign the truth value. Seems to me that People of Faith often speak of "certainty" in their convictions. --DavidLucifer