Digital Companion to C. S. Peirce
1904 | Sketch of Dichotomic Mathematics | NEM 4:289

Any Theorem (as I shall use this term) would be a proposition pronouncing, in effect, that were a general condition which it describes fulfilled, a certain result which it describes in a general way, except so far as it may refer to some object or set of objects supposed in the condition, will be impossible, this proposition being capable of demonstration from propositions previously established, but not without imagining something more than what the condition supposes to exist; and any such proposition would be a Theorem.

1904 [c.] | New Elements (Kaina stoiceia) | EP 2:303

A theorem, as I shall use the word, is an inference obtained by constructing a diagram according to a general precept, and after modifying it as ingenuity may dictate, observing in it certain relations, and showing that they must subsist in every case, retranslating the proposition into general terms.

1908 | Some Amazing Mazes | CP 4.613

I shall term the step of so introducing into a demonstration a new idea not explicitly or directly contained in the premisses of the reasoning or in the condition of the proposition which gets proved by the aid of this introduction, a theoric step. [—] Now to propositions which can only be proved by the aid of theoric steps (or which, at any rate, could hardly otherwise be proved), I propose to restrict the application of the hitherto vague word “theorem,” calling all others, which are deducible from their premisses by the general principles of logic, by the name of corollaries.