A unit of volume, equivalent to eight pints
Exactly 4.54609 liters; an imperial gallon
231 cubic inches or approximately 3.785 liters for liquids (a "U.S. liquid gallon")
One-eighth of a U.S. bushel or approximately 4.405 liters for dry goods (a "U.S. dry gallon").
(in the plural) A large quantity (of any liquid).