radix -
The number of distinct symbols used to represent numbers in a particular base, as 10 for decimal.
Wikipedia
The radix or base refers to the number b in an expression of the form b. The number n is called the exponent and the expression is known formally as exponentiation of b by n or the exponential of n with base b. It is more commonly expressed as "the nth power of b", "b to the nth power" or "b to the power n".