Tautology is popularly defined two main ways: 1) An argument that derives its conclusion from one of its premises, or 2) logical statements that are necessarily true, as in (A∨~A). How are these two definitions reconciled? The second definition is only a statement; it has no premises or conclusions.
You've definitely put your finger on a problem. I'd say that for most purposes the two definitions aren't reconcilable because they belong to different discourses or contexts. The first usage is more colloquial and rhetorical. The second is a technical definition. The term "valid" is used in similarly different ways. In common discourse, one can make a "valid point"; but in technical terms only arguments or inferences, not points, can be valid. There is, however, at least one way to make the two definitions consistent: assume in the first that the premise from which the conclusion is derived is the same as the conclusion. From two premises A and B, the conclusion A follows. This, of course, becomes a variant of begging the question. Conversely, I suppose, one could argue that a tautology follows from itself, making the second definition applicable to an argument. (Note that the way you've phrased the first definition is a bit odd, since all good arguments derive their conclusions from their...
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