Source : Webster's Revised Unabridged Dictionary (1913)

   3. A table for facilitating reference to topics, names, and
      the like, in a book; -- usually alphabetical in
      arrangement, and printed at the end of the volume.

   4. A prologue indicating what follows. [Obs.] --Shak.

   5. (Anat.) The second digit, that next pollex, in the manus,
      or hand; the forefinger; index finger.

   6. (Math.) The figure or letter which shows the power or root
      of a quantity; the exponent. [In this sense the plural is
      always {indices}.]

   {Index error}, the error in the reading of a mathematical
      instrument arising from the zero of the index not being in
      complete adjustment with that of the limb, or with its
      theoretically perfect position in the instrument; a
      correction to be applied to the instrument readings equal
      to the error of the zero adjustment.

   {Index expurgatorius}. [L.] See {Index prohibitorius}
      (below).

   {Index finger}. See {Index}, 5.

   {Index glass}, the mirror on the index of a quadrant,
      sextant, etc.

   {Index hand}, the pointer or hand of a clock, watch, or other
      registering machine; a hand that points to something.

   {Index of a logarithm} (Math.), the integral part of the
      logarithm, and always one less than the number of integral
      figures in the given number. It is also called the
      {characteristic}.

   {Index of refraction}, or {Refractive index} (Opt.), the
      number which expresses the ratio of the sine of the angle
      of incidence to the sine of the angle of refraction. Thus
      the index of refraction for sulphur is 2, because, when
      light passes out of air into sulphur, the sine of the
      angle of incidence is double the sine of the angle of
      refraction.

   {Index plate}, a graduated circular plate, or one with
      circular rows of holes differently spaced; used in
      machines for graduating circles, cutting gear teeth, etc.
      

   {Index prohibitorius} [L.], or {Prohibitory index} (R. C.
      Ch.), a catalogue of books which are forbidden by the
      church to be read; the index expurgatorius [L.], or
      expurgatory index, is a catalogue of books from which
      passages marked as against faith or morals must be removed
      before Catholics can read them. These catalogues are
      published with additions, from time to time, by the
      Congregation of the Index, composed of cardinals,
      theologians, etc., under the sanction of the pope. --Hook.

   {Index rerum} [L.], a tabulated and alphabetized notebook,
      for systematic preservation of items, quotations, etc.