a British imperial capacity measure (liquid or dry) equal to 4 quarts or 4.545 liters
"The recipe called for half a gallon of milk, which is significantly more than the American liquid gallon used in my previous cooking attempts."
A unit of volume, equivalent to eight pints
"The recipe calls for exactly one gallon of milk, which is equivalent to eight pints."
In plain English: A gallon is an American unit of volume that holds about 3.8 liters, roughly enough to fill four large water bottles.
"I need to buy two gallons of milk for this week's meals."
Usage: Use gallon as a countable noun when referring to specific containers or quantities, such as two gallons of milk. Remember that the US liquid gallon differs in size from the imperial gallon used in some other countries.
A surname.
"The local historian wrote an article about Mr. Gallon, a prominent family in the county."
The word gallon comes from the Old Northern French term for a liquid measure. It likely originated in Vulgar Latin as galla, meaning "vessel," and may have roots in an ancient Gaulish language referring to a goblet.