In logic, a tautology is a formula or assertion that is true in every possible interpretation). It is a key concept in propositional logic, where a tautology is defined as a propositional formula that is true under any possible Boolean valuation of its propositional variables). A tautology can also be a compound statement in mathematics that always results in a true statement, regardless of individual statements.

A tautology is different from a logical truth, which is a statement that is true in virtue of its logical form. A logical truth is true in all possible worlds, whereas a tautology is true in all possible interpretations. In other words, a tautology is a formula that is true under any assignment of truth values to its simple components, while a logical truth is true regardless of the meaning of its simple components.

In summary, a tautology is a formula or compound statement that is true in every possible interpretation or assignment of truth values to its simple components, while a logical truth is a statement that is true in virtue of its logical form and is true in all possible worlds.