The Free Dictionary offers the best definition of math in math terms which is "a group of related sciences, including algebra, geometry, and calculus, concerned with the study of number, quantity, shape, and space and their interrelationships by using a specialized notation."

Gallon is a measure of volume, equal to 3.785 liters. Gallon is a measure of volume, equal to 3.785 liters. Gallon is a measure of volume, equal to 3.785 liters. Gallon is a measure of volume, equal to 3.785 liters.

It is the volume of a cube whose sides are 1 decimetre (10 cm). A litre was defined in terms of the volume of a kilogram of water but that definition was abandoned around 50 years ago.

An expression is a collection of terms which are separated by addition or subtraction symbols. Often it represents one side of an equation or a formula.