Full Idea
Usually a truth is only called a 'theorem' when it has not merely been obtained by inference, but is used in turn as a premise for a number of inferences in the science. ….Proofs use non-theorems, which only occur in that proof.
Gist of Idea
'Theorems' are both proved, and used in proofs
Source
Gottlob Frege (Logic in Mathematics [1914], p.204)
Book Reference
Frege,Gottlob: 'Posthumous Writings', ed/tr. Hermes/Long/White etc [Blackwell 1979], p.204