A Hamel basis, or algebraic basis, is a subset of a vector space such that every vector can be uniquely written as a finite linear combination of the basis. Thus, this agrees with the usual notion of basis in a finite-dimensional space. Assuming the axiom of choice, every vector space has a Hamel basis, but in an infinite-dimensional Banach space a Hamel basis is necessarily uncountable. Compare Schauder basis.