(1):

(a.) Having reason, or the faculty of reasoning; endowed with reason or understanding; reasoning.

(2):

(a.) Relating to the reason; not physical; mental.

(3):

(a.) Agreeable to reason; not absurd, preposterous, extravagant, foolish, fanciful, or the like; wise; judicious; as, rational conduct; a rational man.

(4):

(a.) Expressing the type, structure, relations, and reactions of a compound; graphic; - said of formulae. See under Formula.

(5):

(n.) A rational being.