gal ‧ lon /ˈɡælən/ BrE AmE noun [countable]
[ Date: 1200-1300 ; Language: Old North French ; Origin: Medieval Latin galeta 'liquid container, liquid measure' ]
1 . ( written abbreviation gal ) a unit for measuring liquids, equal to eight ↑ pint s . In Britain this is 4.55 litres, and in the US it is 3.79 litres:
a 20 gallon fish tank
The car does about 50 miles per gallon.
2 . informal a lot of a liquid:
We drank gallons of coffee.