An ontology is a formal model of knowledge of the world, where concepts, properties and relations in the real world are defined in some knowledge representation language.
Machine translation is one of the many possible applications of ontologies.
In this setting, they are commonly used for representing the semantics of lexical items, which is given by means of mapping words to ontological concepts. Ontologies used for this purpose are typically built for a particular system that handles only a small set of languages and are not easily extensible for new languages. A Critique of Pure Transfer - Carbonell, Mitamura et al. Doc 3 Extended Objects - Rettig, Simons et al.