The paper investigates technical meta-logical notions and theorems relating to definitions and relative definability and shows how these are relevant to the concerns of knowledge representation. In particular they can be used to identify sets of primitives that are sufficent to fully describe a given domain. Fundamental definability theorems of Tarski are examined, and it is shown how these can...

Two important classic results about modal expressivity are the Characterization and Definability theorems. We develop a general theory for modal logics below first order (in terms of expressivity) which exposes the following result: Characterization and Definability theorems hold for every (reasonable) modal logic whose ω-saturated models have the Hennessy-Milner property. The results are prese...