1 based on clear, practical or scientific reasons: It can't have just disappeared! There must be a perfectly rational explanation.2 sensible and able to make decisions based on intelligent thinking rather than on emotion: rational behaviour | We should be able to sort this out like rational human beings!— rationally adverb: Scientific training makes you think rationally. — rationality noun (U) —opposite irrational
Longman dictionary of contemporary English. 2004.