An Etymological Dictionary of Astronomy and Astrophysics

1. A basis or cause, as for some belief, action, fact, event, etc.
   

2. A statement presented in justification or explanation of a belief or action.
   

3. The mental powers concerned with forming conclusions, judgments, or → inferences especially in orderly rational ways.
   Philo.: The intellectual ability to apprehend the truth cognitively, either immediately in intuition, or by means of a process of inference. See also → wisdom.
   

4. (v.) To think or argue in a logical manner; to form conclusions, judgments, or inferences from facts or premises.

Etymology (EN): M.E. resoun, reisun (n.), from O.Fr. reisun, reson, raison, from L. rationem (nom. ratio) “reckoning, understanding, motive, cause,” from ratus, p.p. of reor, reri “to reckon, think;” cognate with Pers. râ, rây “reason,” as below; O.C.S. raditi “to take thought, attend to,” O.Ir. im-radim “to deliberate, consider.”

Etymology (PE): Râyan, from rây + -an. The first component rây, râ “because of, by reason of,” in barây-e “because of,” cerâ “for what reason, why?,” irâ “for this reason, therefore,” zirâ “because, on account of;” also rây “opinion, consult;” râymand “reasonable, wise;” râyzan “a person whom one consults; wise;” Mid.Pers. râd, rây “because of, for the sake of, on behalf of;” O.Pers. rā “reason, cause,” in rādiy “for this reason;” Av. rādah- “generosity, care, generous,” rāsta- “to do right, to fit;” cf. Skt. rādh- “to succeed, be successful;” Gk. arithmos “number, amount;” L. ratio, as above; PIE base *rē-, *rə-. The second component -an a suffix as in rowzan, rowšan, suzan, rasan, zaqan, hâvan, etc.