Haar space

(approximation theory) A finite-dimensional subspace V of 𝒞(X, 𝕂), where X is a compact space and 𝕂 either the real numbers or the complex numbers, such that for any given f∈𝒞(X, 𝕂) there is exactly one element of V that approximates f "best", i.e. with minimum distance to f in supremum norm.