1. In modal logic, a formula $\phi$ that serves to characterize some class of Kripke frames. It is said that $\phi$ is canonical for a class $S$ of Kripke frames if:

1. 1 $\phi$ is true in every point in every Kripke model built on a frame in $S$

2. 2 for any normal modal logic counting $\phi$ as a theorem, the frame of its canonical model is a member of $S$

An analogous definition holds for sets of formula $\Gamma$.

2. A formula (or axiom scheme) $\phi$ in modal logic with the property that for any normal modal logic $\text{L}$ for which all instances of $\phi$ are theorems, $\phi$ is valid on the frame of the canonical model of $\text{L}$.