Source : Webster's Revised Unabridged Dictionary (1913)

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

Rational \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.

Source : WordNet®

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

Source : Free On-Line Dictionary of Computing

rational
     
        [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}.