Comment by tptacek

For whatever it's worth --- probably not much if you don't do any cryptography engineering --- the definition provided here is of a password KDF. There are other important kinds of KDF, and they don't necessarily have the goal of stretching an untenably low-entropy secret into a facsimile of an actual key. HKDF, for instance, is often the way you extract keys from a Diffie-Hellman exchange.