The   $\sqrt{\hskip 8.535827pt}$  is called a radical. The of the radical symbol is the first letter “r” of the Latin radix, which “root (http://planetmath.org/NthRoot)”.
There is other terminology that is related to radicals: In the expression $\sqrt[3]{x}$, $x$ is the radicand and $3$ is the index.
One can use fractional powers instead of the radical symbol via $\sqrt[n]{x}=x^{\frac{1}{n}}$. This practice of avoiding the radical symbol may be due to typesetting issues.