Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

Source : WordNet®

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