A digit is a character from a character set provided for the respective number system.
In the decimal system these are the digits 0 to 9, in the hexadecimal system the digits 0 to 9 and the letters A, B, C, D, E, F, in the binary system the "0" and the "1".
A number is represented by several digits. In the case of longer numbers, they are grouped together and can be designated accordingly. The groups can be powers to the base of a number. Another group designation is based on the prefixes, which represent the powers of ten by upper and lower case letters.
For example, the number 1234000 can be represented as `1.234 * 10^6` or as 1.234 million, whose prefix is the mega (M).