One of the characters used to indicate a whole number (unit) in a numbering system. In any numbering system, the number of possible digits is equal to the base, or radix, used. For example, the decimal (base-10) system has 10 digits, 0 through 9; the binary (base-2) system has 2 digits, 0 and 1; and the hexadecimal (base-16) system has 16 digits, 0 through 9 and A through F.