1. [adj]  having its source in or being guided by the intellect (distinguished from experience or emotion); "a rational analysis"
2. [adj]  of or associated with or requiring the use of the mind; "intellectual problems"; "the triumph of the rational over the animal side of man"
3. [adj]  consistent with or based on or using reason; "rational behavior"; "a process of rational inference"; "rational thought"
4. [adj]  (math) capable of being expressed as a quotient of integers; "rational numbers"

1. \Ra"tion*al\, a. [L. rationalis: cf. F. rationnel. See
   {Ratio}, {Reason}, and cf. {Rationale}.]
   1. Relating to reason; not physical; mental.
            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.
   2. Having reason, or the faculty of reasoning; endowed with
      reason or understanding; reasoning.
            It is our glory and happiness to have a rational
            nature.                               --Law.
   3. Agreeable to reason; not absurd, preposterous,
      extravagant, foolish, fanciful, or the like; wise;
      judicious; as, rational conduct; a rational man.
   4. (Chem.) Expressing the type, structure, relations, and
      reactions of a compound; graphic; -- said of formul[ae].
      See under {Formula}.
   {Rational horizon}. (Astron.) See {Horizon}, 2
      (b) .
   {Rational quantity} (Alg.), one that can be expressed without
      the use of a radical sign, or in extract 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.
   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
          traditional; 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 prospect of
          success.
                What higher in her society thou find'st
                Attractive, human, rational, love still.
                                                  --Milton.
                A law may be reasonable in itself, although a
                man does not allow it, or does not know the
                reason of the lawgivers.          --Swift.
   

2. \Ra"tion*al\, n.
   A rational being. --Young.
   

[Mathematics] a fractional number n/d, where n and d are integers, n is the numerator and d is the denominator. The set of all rational numbers is usually called Q. Computers do not usually deal with rational numbers but instead convert them to real numbers which are represented (approximately in some cases) as floating-point numbers. Compare irrational.