The Collaborative International Dictionary of English v.0.48:

Rational \Ra"tion*al\ (r[a^]sh"[u^]n*al), a. [L. rationalis: cf.
   F. rationnel. See Ratio, Reason, and cf. Rationale.]
   1. Relating to the reason; not physical; mental.
      [1913 Webster]

            Moral philosophy was his chiefest end; for the
            rational, the natural, and mathematics . . . were
            but simple pastimes in comparison of the other.
                                                  --Sir T.
                                                  North.
      [1913 Webster]

   2. Having reason, or the faculty of reasoning; endowed with
      reason or understanding; reasoning.
      [1913 Webster]

            It is our glory and happiness to have a rational
            nature.                               --Law.
      [1913 Webster]

   3. Agreeable to reason; not absurd, preposterous,
      extravagant, foolish, fanciful, or the like; wise;
      judicious; as, rational conduct; a rational man.
      [1913 Webster]

   4. (Chem.) Expressing the type, structure, relations, and
      reactions of a compound; graphic; -- said of formulae. See
      under Formula.
      [1913 Webster]

   Rational horizon. (Astron.) See Horizon, 2
      (b) .

   Rational quantity (Alg.), one that can be expressed without
      the use of a radical sign, or in exact parts of unity; --
      opposed to irrational or radical quantity.

   Rational symptom (Med.), one elicited by the statements of
      the patient himself and not as the result of a physical
      examination.
      [1913 Webster]

   Syn: Sane; sound; intelligent; reasonable; sensible; wise;
        discreet; judicious.

   Usage: Rational, reasonable. Rational has reference to
          reason as a faculty of the mind, and is opposed to
          irrational; as, a rational being, a rational state of
          mind, rational views, etc. In these cases the
          speculative reason is more particularly, referred to.
          Reasonable has reference to the exercise of this
          faculty for practical purposes, and means, governed or
          directed by reason; as, reasonable desires or plans; a
          reasonable charge; a reasonable prospect of success.
          [1913 Webster]

                What higher in her society thou find'st
                Attractive, human, rational, love still.
                                                  --Milton.
          [1913 Webster]

                A law may be reasonable in itself, although a
                man does not allow it, or does not know the
                reason of the lawgivers.          --Swift.
          [1913 Webster]