Definition of "gallon"

A unit of volume, equivalent to eight pints  examples

(Britain, Canada) exactly 4.54609 liters; an imperial gallon  examples

(US) 231 cubic inches or approximately 3.785 liters for liquids (a "U.S. liquid gallon")  examples

(US) one-eighth of a U.S. bushel or approximately 4.405 liters for dry goods (a "U.S. dry gallon").  examples

(in the plural, informal) A large quantity (of any liquid).  examples