1. The property attributed to a modal logic $\text{L}$ with a unary necessity operator when every instance of the axiom $\text{K}$, i.e., $\square \left(\phi \to \psi \right)\to \left(\square \phi \to \square \psi \right)$, is an $\text{L}$-theorem and rule necessitation, i.e., the rule that $\text{L}$-theoremhood of any $\phi$ entails $\text{L}$-theoremhood of $\square \phi$, is an admissible inference.

2. In Kripke semantics for modal logic, describes a possible world at which formulae of the form $\square \phi$ are given the standard Leibnizian reading, so that the truth condition for formulae $\square \phi$ is described by the clause:

• • $\square \phi$ is true at $w$ if for all worlds $w^{\prime }$ accessible from $w$, $\phi$ is true at $w^{\prime }$.