Haar space

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