bivalence

In logic, a principle or law that can be formulated as ‘every proposition is either true or false’.

If the principle of bivalence is true, then two-valued logic, in which true and false are in practice the two main truth-values of propositions, is the only possible logic. If the principle is false, then many-valued logics are possible, in which propositions can have values such as ‘known to be false’, ‘known to be true’, ‘necessarily false’, and ‘necessarily true’.

‘Every proposition is either true or false’ is also one form of what is known as the law of the excluded middle.