Source : Webster's Revised Unabridged Dictionary (1913)

Ratio \Ra"ti*o\, n. [L., fr. reri, ratus, to reckon, believe,
   think, judge. See {Reason}.]
   1. (Math.) The relation which one quantity or magnitude has
      to another of the same kind. It is expressed by the
      quotient of the division of the first by the second; thus,
      the ratio of 3 to 6 is expressed by 3/6 or 1/2; of a to b
      by a/b; or (less commonly) the second is made the
      dividend; as, a:b = b/a.

   Note: Some writers consider ratio as the quotient itself,
         making ratio equivalent to a number. The term ratio is
         also sometimes applied to the difference of two
         quantities as well as to their quotient, in which case
         the former is called arithmetical ratio, the latter,
         geometrical ratio. The name ratio is sometimes given to
         the rule of three in arithmetic. See under {Rule}.

   2. Hence, fixed relation of number, quantity, or degree;
      rate; proportion; as, the ratio of representation in
      Congress.

   {Compound ratio}, {Duplicate ratio}, {Inverse ratio}, etc.
      See under {Compound}, {Duplicate}, etc.

   {Ratio of a geometrical progression}, the constant quantity
      by which each term is multiplied to produce the succeeding
      one.