Number

A number is a unit, or a collection of units. For example, one dollar is a unit; five dollars is a collection of units.

Numbers may be either abstract or concrete. A concrete number is one applied to a particular unit, such as 5 men, 6 horses, and 9 dollars. An abstract number is one not applied to a particular unit, such as 5, 6, and 9.

Similar numbers are numbers which have the same unit, such as 6 boys, 8 boys, and 10 boys.