In chemistry, a radical is a group of atoms, either in a compound or existing alone in which all the atomic valences are not satisfied by chemical bonding. Thus, removal of a hydrogen atom from methane, CH₄, gives the methyl radical CH₃. The term is usually taken to mean a free radical, that is, one existing free for a short time in a reaction.

In mathematics, a radical is the root of a number or quantity. The radical sign is √, or more generally ⁿ √. It seems to have been first used in 1525 by Christoff Rudolff (1499–1545) in his Die Coss. Another way of expressing the nth of a number x^1/n.