Page "Soundness" Paragraph 17

Soundness of a deductive system is the property that any sentence that is provable in that deductive system is also true on all interpretations or structures of the semantic theory for the language upon which that theory is based.

In symbols, where S is the deductive system, L the language together with its semantic theory, and P a sentence of L: if ⊢< sub > S </ sub > P, then also ⊨< sub > L </ sub > P.