What is the most specific word that describes both? As in "all theorems and conjectures are ..." ?
The term "Mathematical statements" is perhaps slightly too general for many intended purposes. Though theorems and conjectures form a subset of all mathematical statements, theorems must be true and we hope for conjectures to be true, while there are many mathematical statements which are false. This distinction is very far from minor (for a mathematician), however due to the inherent difference between a theorem and a conjecture, for a more precise description you would definitely have to sacrifice conciseness.