A radix (also called a base) is the number of unique digits, including zero, that a positional number system uses to represent numbers.
For example, the decimal system uses the digits 0 through 9, so its radix is 10. (In math, we often start counting at 0 because we need to be able to represent nothing.)
1. 0
2. 1
3. 2
4. 3
5. 4
6. 5
7. 6
8. 7
9. 8
10. 9