When a number is raised to the power of a fraction, that fraction is the number's rational exponent. Such an expression is equivalent to a root.

Notice that the denominator of the rational exponent gives the index of the root, while the numerator gives the power to which $a$ is raised. Since $n$ is a denominator, it cannot be zero. Moreover, if $n$ is an even number, then $a^m$ must be non-negative.