What are ontologies and if so, how many?

In many projects we notice that the demand for ontologies in the terminology environment is increasing. But why actually? What are ontologies? How can terminology and ontology be combined? Do we need ontologies - and if so: one or more? We have addressed these questions in our article "The use of term systems in terminology management" in the current June issue of "edition - Die Fachzeitschrift für Terminologie".