Radix
Radix is a Latin word for "root". Root can be considered a synonym for base in the arithmetical sense.
It is the number of different digits you can use to count. For example, when you count with your fingers, you use ten different numbers: 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9. So, the radix is 10. It's like having a set of counting tools!