An * interpretation* of a propositional
logic formula is an
assignment of a value (true or false) to each atom.
There are *2 ^{n}* possible interpretations of a formula with

Terminology:

- A formula is
*valid*if it is true under every possible interpretation. Otherwise, it is*invalid*. - A formula is
*consistent*or*satisfiable*if it is true under some interpretation. If it is false under every interpretation, it is*inconsistent*or*unsatisfiable*.

If a formula *F* is true under an interpretation *I*, *I* is a * model*
for *F*.

Two formulas *F* and *G* are * equivalent* if they have the same values
under every interpretation: * F &harr G *.