radical sign

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.
• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a sign indicating the extraction of a root