The Collaborative International Dictionary of English v.0.48:

Imperial \Im*pe"ri*al\, a. [OE. emperial, OF. emperial, F.
   imp['e]rial, fr. L. imperialis, fr. imperium command,
   sovereignty, empire. See Empire.]
   1. Of or pertaining to an empire, or to an emperor; as, an
      imperial government; imperial authority or edict.
      [1913 Webster]

            The last
            That wore the imperial diadem of Rome. --Shak.
      [1913 Webster]

   2. Belonging to, or suitable to, supreme authority, or one
      who wields it; royal; sovereign; supreme. "The imperial
      democracy of Athens." --Mitford.
      [1913 Webster]

            Who, as Ulysses says, opinion crowns
            With an imperial voice.               --Shak.
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            To tame the proud, the fetter'd slave to free,
            These are imperial arts, and worthy thee. --Dryden.
      [1913 Webster]

            He sounds his imperial clarion along the whole line
            of battle.                            --E. Everett.
      [1913 Webster]

   3. Of superior or unusual size or excellence; as, imperial
      paper; imperial tea, etc.
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   Imperial bushel, gallon, etc. See Bushel, Gallon,
      etc.

   Imperial chamber, the, the sovereign court of the old
      German empire.

   Imperial city, under the first German empire, a city having
      no head but the emperor.

   Imperial diet, an assembly of all the states of the German
      empire.

   Imperial drill. (Manuf.) See under 8th Drill.

   Imperial eagle. (Zool.) See Eagle.

   Imperial green. See Paris green, under Green.

   Imperial guard, the royal guard instituted by Napoleon I.


   Imperial weights and measures, the standards legalized by
      the British Parliament.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Imperial \Im*pe"ri*al\, n. [F. imp['e]riale: cf. Sp. imperial.]
   [1913 Webster]
   1. The tuft of hair on a man's lower lip and chin; -- so
      called from the style of beard of Napoleon III.
      [1913 Webster]

   2. An outside seat on a diligence. --T. Hughes.
      [1913 Webster]

   3. A luggage case on the top of a coach. --Simmonds.
      [1913 Webster]

   4. Anything of unusual size or excellence, as a large
      decanter, a kind of large photograph, a large sheet of
      drawing, printing, or writing paper, etc.
      [1913 Webster]

   5. A gold coin of Russia worth ten rubles, or about eight
      dollars. --McElrath.
      [1913 Webster]

   6. A kind of fine cloth brought into England from Greece. or
      other Eastern countries, in the Middle Ages.
      [1913 Webster]

   7. A game at cards differing from piquet in some minor
      details, and in having a trump; also, any one of several
      combinations of cards which score in this game.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

imperial
    adj 1: relating to or associated with an empire; "imperial
           colony"; "the imperial gallon was standardized legally
           throughout the British Empire"
    2: of or belonging to the British Imperial System of weights and
       measures
    3: befitting or belonging to an emperor or empress; "imperial
       palace"
    4: belonging to or befitting a supreme ruler; "golden age of
       imperial splendor"; "purple tyrant"; "regal attire"; "treated
       with royal acclaim"; "the royal carriage of a stag's head"
       [syn: imperial, majestic, purple, regal, royal]
    n 1: a small tufted beard worn by Emperor Napoleon III [syn:
         imperial, imperial beard]
    2: a piece of luggage carried on top of a coach

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "imperial":
   Vandyke, absolute, absolutist, absolutistic, arbitrary,
   aristocratic, arrogant, august, authoritarian, authoritative,
   autocratic, beard, beaver, bossy, bristles, crown, despotic,
   dictatorial, domineering, down, duodecimo, dynastic, eighteenmo,
   exalted, excellent, feudal, folio, goatee, grand, grinding,
   high-handed, imperative, imperatorial, imperatorious, imperious,
   imposing, kinglike, kingly, lofty, lordly, magisterial, magistral,
   magnificent, majestic, masterful, medium, monarchal, monarchial,
   monarchic, monocratic, noble, octavo, octodecimo, oppressive,
   overbearing, overruling, peach fuzz, peremptory, princelike,
   princely, purple, quarto, queenlike, queenly, regal, repressive,
   royal, severe, sextodecimo, side whiskers, sixteenmo, sovereign,
   splendid, strict, stubble, super, superior, suppressive, supreme,
   tuft, twelvemo, tyrannical, tyrannous, whiskers



U.S. Gazetteer Counties (2000):

Imperial -- U.S. County in California
   Population (2000):    142361
   Housing Units (2000): 43891
   Land area (2000):     4174.732434 sq. miles (10812.506906 sq. km)
   Water area (2000):    306.998053 sq. miles (795.121274 sq. km)
   Total area (2000):    4481.730487 sq. miles (11607.628180 sq. km)
   Located within:       California (CA), FIPS 06
   Location:             32.963098 N, 115.487593 W
   Headwords:
    Imperial
    Imperial, CA
    Imperial County
    Imperial County, CA


U.S. Gazetteer Places (2000):

Imperial, MO -- U.S. Census Designated Place in Missouri
   Population (2000):    4373
   Housing Units (2000): 1720
   Land area (2000):     5.382812 sq. miles (13.941418 sq. km)
   Water area (2000):    0.741433 sq. miles (1.920303 sq. km)
   Total area (2000):    6.124245 sq. miles (15.861721 sq. km)
   FIPS code:            34354
   Located within:       Missouri (MO), FIPS 29
   Location:             38.370735 N, 90.373460 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Imperial, MO
    Imperial


U.S. Gazetteer Places (2000):

Imperial, NE -- U.S. city in Nebraska
   Population (2000):    1982
   Housing Units (2000): 887
   Land area (2000):     2.517650 sq. miles (6.520684 sq. km)
   Water area (2000):    0.001206 sq. miles (0.003124 sq. km)
   Total area (2000):    2.518856 sq. miles (6.523808 sq. km)
   FIPS code:            23690
   Located within:       Nebraska (NE), FIPS 31
   Location:             40.518398 N, 101.642491 W
   ZIP Codes (1990):     69033
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Imperial, NE
    Imperial


U.S. Gazetteer Places (2000):

Imperial, CA -- U.S. city in California
   Population (2000):    7560
   Housing Units (2000): 2385
   Land area (2000):     3.912696 sq. miles (10.133835 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    3.912696 sq. miles (10.133835 sq. km)
   FIPS code:            36280
   Located within:       California (CA), FIPS 06
   Location:             32.842630 N, 115.571841 W
   ZIP Codes (1990):     92251
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Imperial, CA
    Imperial


U.S. Gazetteer Places (2000):

Imperial-Enlow, PA -- U.S. Census Designated Place in Pennsylvania
   Population (2000):    3514
   Housing Units (2000): 1492
   Land area (2000):     4.040907 sq. miles (10.465900 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    4.040907 sq. miles (10.465900 sq. km)
   FIPS code:            36772
   Located within:       Pennsylvania (PA), FIPS 42
   Location:             40.451404 N, 80.242353 W
   ZIP Codes (1990):     15126
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Imperial-Enlow, PA
    Imperial-Enlow
    Imperial, PA
    Imperial


U.S. Gazetteer Places (2000):

Imperial, TX -- U.S. Census Designated Place in Texas
   Population (2000):    428
   Housing Units (2000): 245
   Land area (2000):     4.233356 sq. miles (10.964340 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    4.233356 sq. miles (10.964340 sq. km)
   FIPS code:            35828
   Located within:       Texas (TX), FIPS 48
   Location:             31.271968 N, 102.695799 W
   ZIP Codes (1990):     79743
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Imperial, TX
    Imperial