Haar space

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