Algebraic closure refers to the concept in algebra that a field contains all the algebraic numbers necessary to solve any polynomial equation. In other words, an algebraically closed field is a field in which every non-constant polynomial equation has a root. Formally, a field F is said to be algebraically closed if every non-constant polynomial with coefficients in F has a root in F. This means that for any polynomial equation of the form P(x) = 0, where P(x) is a non-constant polynomial with coefficients in F, there exists at least one element x in F that satisfies the equation. The concept of algebraic closure is important in various areas of mathematics, including abstract algebra and complex analysis. The most well-known example of an algebraically closed field is the set of complex numbers, denoted by C.