radical, in mathematics, symbol () placed over a number or expression, called the radicand, to indicate a root of the radicand. When used without a sign or index number, as in 4, it designates the positive square root of the radicand, i.e., 2. If both square roots are meant, the radical sign is preceded by ±, as in ±4. To indicate higher roots of the radicand, e.g., cube or fourth roots, an index number is used, as in 327. The radical sign is generally taken to indicate the principal root of the radicand, i.e., 327 = 3, although any radicand will have n different nth roots. The term radical is sometimes used loosely to refer to the entire expression consisting of radical sign and radicand.

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