A square root of a real number a is a number x such that x² = a. If a is negative, there is no such real number. If a is positive, there are two such numbers, one positive and one negative. For a ≥ 0, the notation $\sqrt{a}$ is used to denote quite specifically the non-negative square root of a. For complex numbers, square root is a multifunction so that a branch needs to be chosen.