In arithmetic, the radix or base refers to the number b in an expression of the form bn. 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". The term power strictly refers to the entire expression, but is sometimes used to refer to the exponent.

When b is an integer bigger than 1, this process is particularly important in positional numeral systems for denoting numbers. For a given integer b the positional numeral system is called base b.

In general, b and n can be arbitrary real or complex numbers.

The inverse function to exponentiation with base b (when it is well-defined) is called the logarithm with base b, denoted logb. Thus logb(bn) = n.