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A number is an abstract idea used in counting and measuring. A symbol which represents a number is called a numeral, but in common usage the word number is used for both the idea and the symbol. In addition to their use in counting and measuring, numerals are often used for labels (telephone numbers), for ordering (serial numbers), and for codes (ISBNs). In mathematics, the definition of a number has been extended to include new kinds of numbers, such as rational numbers, irrational numbers, zero, negative numbers, and complex numbers.