Tautology

In propositional logic a tautology is a formula that is always true regardless of the truth assignment. A tautological formula is also called a valid formula.

Example

Consider the propositional formula A ∨ ¬A. The truth table below shows this to be always true regardless of the truth value of A.

A	¬A	A ∨ ¬A
t	f	t
f	t	t

References

Doets, Kees. From Logic to Logic Programming. MIT Press, 1994.