# Definition:Tautology/Formal Semantics/Predicate Logic

## Definition

Let $\mathbf A$ be a WFF of predicate logic.

Then $\mathbf A$ is a tautology if and only if:

$\mathcal A, \sigma \models_{\mathrm{PL_A}} \mathbf A$

for every structure $\mathcal A$ and assignment $\sigma$.

That $\mathbf A$ is a tautology can be denoted as:

$\models_{\mathrm{PL_A}} \mathbf A$

## Also known as

In this context, tautologies are also referred to as (logically) valid formulas.

However, on $\mathsf{Pr} \infty \mathsf{fWiki}$, this can easily be confused with a formula that is valid in a single structure, and is therefore discouraged.

## Also denoted as

When the formal semantics under discussion is clear from the context, $\models \phi$ is a common shorthand for $\models_{\mathrm{PL_A}} \phi$.