Full Idea
If every proof-theoretically valid inference is semantically valid (so that |- entails |=), the proof theory is said to be 'sound'.
Gist of Idea
A proof theory is 'sound' if its valid inferences entail semantic validity
Source
Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], 1.1.7)
Book Reference
Enderton,Herbert B.: 'A Mathematical Introduction to Logic' [Academic Press 2001], p.2