Haar space

In approximation theory, a Haar space or Chebyshev space is 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 $f\in 𝒞(X,𝕂)$ there is exactly one element of $V$ that approximates $f$ "best", i.e. with minimum distance to $f$ in supremum norm.