Definition of digit

A digit is any of the basic symbols in a numeration system that are used to write a number. 

In our decimal or base-ten numeration system, the 10 digits that are used to represent any number are 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9.

Examples

The number  4563 has 4 digits and these are 4, 5, 6, 3.

The number 6 has only 1 digit and it is 6.

There are other types of digits that are being used in other numeration systems such as in Chinese numeration system , Easter Arabic numeration system, Roman numeration system, and Mayan numeration system.

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