The Collaborative International Dictionary of English v.0.48:

Gallon \Gal"lon\, n. [OF galon, jalon, LL. galo, galona, fr.
   galum a liquid measure; cf. F. jale large bowl. Cf. Gill a
   measure.]
   A measure of capacity, containing four quarts; -- used, for
   the most part, in liquid measure, but sometimes in dry
   measure.
   [1913 Webster]

   Note: The standart gallon of the Unites States contains 231
         cubic inches, or 8.3389 pounds avoirdupois of distilled
         water at its maximum density, and with the barometer at
         30 inches. This is almost exactly equivalent to a
         cylinder of seven inches in diameter and six inches in
         height, and is the same as the old English wine gallon.
         The beer gallon, now little used in the United States,
         contains 282 cubic inches. The English imperial gallon
         contains 10 pounds avoirdupois of distilled water at
         62? of Fahrenheit, and barometer at 30 inches, equal to
         277.274 cubic inches.
         [1913 Webster]

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.
      [1913 Webster]

   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]

WordNet (r) 3.0 (2006):

gallon
    n 1: United States liquid unit equal to 4 quarts or 3.785 liters
         [syn: gallon, gal]
    2: a British imperial capacity measure (liquid or dry) equal to
       4 quarts or 4.545 liters [syn: gallon, Imperial gallon,
       congius]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

GALLON, measures. A gallon is a liquid measure, containing two hundred and
thirty-one cubic inches, or four quarts.