In mathematics and computer science, a numerical digit is a symbol, e.g. 3, used in numerals (combinations of symbols), e.g. 37, to represent numbers (integers or real numbers) in positional numeral systems. (The name comes from the fact that the 10 fingers correspond to the 10 digits in the common base 10 number system, ie a decimal digit).
Examples of digits include any one of the decimal characters "0" through "9", either of the binary characters "0" or "1", and the digits "0"..."9","A",...,"F" used in the hexadecimal system. In a given number system, if the base (radix) is an integer, the number of needed digits, including zero, is always equal to the absolute value of the base.
(Source: Federal Standard 1037C)