\sqrt

Synopsis, one of:

\sqrt{arg}
\sqrt[root-number]{arg}

The square root, or optionally other roots, of arg. The optional argument root-number gives the root, i.e., enter the cube root of x+y as \sqrt[3]{x+y}. The radical grows with the size of arg (as the height of the radical grows, the angle on the leftmost part gets steeper, until for a large enough arg, it is vertical).

LaTeX has a separate \surd character (see Math symbols).