A number is a mathematical object used to measure, count, & label. In common usage, number may refer to mathematical object, a word or phrase, or a symbol.

The notion of number has been extended over the centuries to include 0, negative numbers, rational numbers such as 2/5 and −1/3, real numbers such as √2 and π, & complex numbers (which extend the real numbers with a square root of −1).

Calculations with numbers are done with arithmetical operations, the most familiar being addition, subtraction, multiplication, division, & exponentiation. Their study or usage is called arithmetic.

The same term may also refer to number theory, the study of the properties of numbers.

Multiple & Factor

Rational & Fraction