Source : Webster's Revised Unabridged Dictionary (1913)

Seal \Seal\, n. [OE. seel, OF. seel, F. sceau, fr. L. sigillum a
   little figure or image, a seal, dim. of signum a mark, sign,
   figure, or image. See {Sign}, n., and cf. {Sigil}.]
   1. An engraved or inscribed stamp, used for marking an
      impression in wax or other soft substance, to be attached
      to a document, or otherwise used by way of authentication
      or security.

   2. Wax, wafer, or other tenacious substance, set to an
      instrument, and impressed or stamped with a seal; as, to
      give a deed under hand and seal.

            Till thou canst rail the seal from off my bond Thou
            but offend;st thy lungs to speak so loud. --Shak.

   3. That which seals or fastens; esp., the wax or wafer placed
      on a letter or other closed paper, etc., to fasten it.

   4. That which confirms, ratifies, or makes stable; that which
      authenticates; that which secures; assurance. ``under the
      seal of silence.'' --Milton.

            Like a red seal is the setting sun On the good and
            the evil men have done.               --Lonfellow.

   5. An arrangement for preventing the entrance or return of
      gas or air into a pipe, by which the open end of the pipe
      dips beneath the surface of water or other liquid, or a
      deep bend or sag in the pipe is filled with the liquid; a
      draintrap.

   {Great seal}. See under {Great}.

   {Privy seal}. See under {Privy}, a.

   {Seal lock}, a lock in which the keyhole is covered by a seal
      in such a way that the lock can not be opened without
      rupturing the seal.

   {Seal manual}. See under {Manual}, a.

   {Seal ring}, a ring having a seal engraved on it, or
      ornamented with a device resembling a seal; a signet ring.
      --Shak.

Great \Great\, a. [Compar. {Greater}; superl. {Greatest}.] [OE.
   gret, great, AS. gre['a]t; akin to OS. & LG. gr[=o]t, D.
   groot, OHG. gr[=o]z, G. gross. Cf. {Groat} the coin.]
   1. Large in space; of much size; big; immense; enormous;
      expanded; -- opposed to small and little; as, a great
      house, ship, farm, plain, distance, length.

   2. Large in number; numerous; as, a great company, multitude,
      series, etc.

   3. Long continued; lengthened in duration; prolonged in time;
      as, a great while; a great interval.

   4. Superior; admirable; commanding; -- applied to thoughts,
      actions, and feelings.

   5. Endowed with extraordinary powers; uncommonly gifted; able
      to accomplish vast results; strong; powerful; mighty;
      noble; as, a great hero, scholar, genius, philosopher,
      etc.

   6. Holding a chief position; elevated: lofty: eminent;
      distingushed; foremost; principal; as, great men; the
      great seal; the great marshal, etc.

            He doth object I am too great of birth. --Shak.

   7. Entitled to earnest consideration; weighty; important; as,
      a great argument, truth, or principle.

   8. Pregnant; big (with young).

            The ewes great with young.            --Ps. lxxviii.
                                                  71.

   9. More than ordinary in degree; very considerable in degree;
      as, to use great caution; to be in great pain.

            We have all Great cause to give great thanks.
                                                  --Shak.

   10. (Genealogy) Older, younger, or more remote, by single
       generation; -- often used before grand to indicate one
       degree more remote in the direct line of descent; as,
       great-grandfather (a grandfather's or a grandmother's
       father), great-grandson, etc.

   {Great bear} (Astron.), the constellation Ursa Major.

   {Great cattle} (Law), all manner of cattle except sheep and
      yearlings. --Wharton.

   {Great charter} (Eng. Hist.), Magna Charta.

   {Great circle of a sphere}, a circle the plane of which
      passes through the center of the sphere.

   {Great circle sailing}, the process or art of conducting a
      ship on a great circle of the globe or on the shortest arc
      between two places.

   {Great go}, the final examination for a degree at the
      University of Oxford, England; -- called also {greats}.
      --T. Hughes.

   {Great guns}. (Naut.) See under Gun.

   {The Great Lakes} the large fresh-water lakes (Lakes
      Superior, Michigan, Huron, Erie, and Ontario) which lie on
      the northern borders of the United States.

   {Great master}. Same as {Grand master}, under {Grand}.

   {Great organ} (Mus.), the largest and loudest of the three
      parts of a grand organ (the others being the choir organ
      and the swell, and sometimes the pedal organ or foot
      keys), It is played upon by a separate keyboard, which has
      the middle position.

   {The great powers} (of Europe), in modern diplomacy, Great
      Britain, France, Germany, Austria, Russia, and Italy.

   {Great primer}. See under {Type}.

   {Great scale} (Mus.), the complete scale; -- employed to
      designate the entire series of musical sounds from lowest
      to highest.

   {Great sea}, the Mediterranean sea. In Chaucer both the Black
      and the Mediterranean seas are so called.

   {Great seal}.
       (a) The principal seal of a kingdom or state.
       (b) In Great Britain, the lord chancellor (who is
           custodian of this seal); also, his office.

Source : WordNet®

great seal
     n : the principal seal of a government, symbolizing authority or
         sovereignty