the sign (√ or ) used before a quantity to indicate that the square root (or a specified root indicated by an imposed index number) is to be extracted: derived from the r in Latin radix, root
See radical sign in American Heritage Dictionary 4
noun Mathematics
The sign √ placed before a quantity, indicating extraction of the root designated by a raised integer. When extracting a square root, the raised integer is customarily omitted.
The radical sign together with a horizontal bar extending from its top to the end of the expression from which a root is to be extracted.