My Own Writing
Rational literally means “having reason.” In the process of belief formation, to be rational means to base one’s beliefs on reasons–either direct observation, valid induction or deduction, or provisional acceptance from a source one has reason to believe is rational and reliable.
From Wikipedia on 11/5/12
From Old French rationel, rational, from Latin rationalis (“of or belonging to reason, rational, reasonable”), from ratio (“reason”).
Logically sound; not contradictory or otherwise absurd.