Statements such as theorems, propositions, and lemmas are all (mostly) proven statements. They differ from, say, definitions by the fact that they describe consequences of axioms, rather than label a new object.
Is there a name for the class of all "truthful statements" that describes each of the 'theorem' types?
This class should consist of the following:
I don't think that conjecture or hypothesis belong in this class since they are results that have yet to be proved.
The term statement by itself seems too broad (taken from the Wiki):
In logic, the term statement is variously understood to mean either:
(a) a meaningful declarative sentence that is true or false, or
(b) the assertion that is made by a true or false declarative sentence.
Similarly, proposition is already in the class, so it would perhaps be improper to call the class "propositions".
I have never heard of these things described as facts, though that feels like an appropriate name.