Evaluate the expression [8]638.

The $n$-th root of $x^m$ can be written as $x^{m/n}$. In this case, $n=8$ and $m=8$.

$$\sqrt[8]{63^8} = 63^{\frac{8}{8}}$$

$$63^1 = 63$$

The evaluated expression is:

$$\text{63}$$