A radical is a mathematical symbol that represents a particular root of a number. The symbol is a square root sign, which is used to indicate the root of a number. The number under the square root sign is called the radicand, and the number outside the sign is called the index or degree. A radical expression is any expression containing a radical, such as a square root, a cube root, or a higher root. If the expression contains no transcendental functions or transcendental numbers, it is called an algebraic expression.

In addition to its mathematical meaning, the term "radical" can also refer to a person who holds or follows strong convictions or extreme principles. It can also describe something that is very different from the usual or traditional, or that favors extreme changes in existing views, habits, conditions, or institutions. In medicine, "radical" can refer to a surgical procedure that is designed to remove the root of a disease or all diseased and potentially diseased tissue.