Definition: a set of concepts and categories in a subject area or domain that shows their properties and the relations between them.
As defined in whatis.com: In general, ontology is the study or concern about what kinds of things exist – what entities there are in the universe. It derives from the Greek onto (being) and logia (written or spoken discourse). It is a branch of metaphysics, the study of first principles or the essence of things.
It does look like a complicated term but it’s not as complex as it looks. What we are trying to do in ontology is to map and relate all the connected things in a domain together.
As referenced from Wikipedia, principal questions of ontology include:
“What can be said to exist?”
“What is a thing?”
“Into what categories, if any, can we sort existing things?”
“What are the meanings of being?”
“What are the various modes of being of entities?”
The questions come from philosophical background. Ontology is a philosophical fundamental. It points towards the study of nature of being, becoming, existence or reality and as well as the basic categories of being and their relations.
Ontologies support conceptualization. They support in bringing out the structure of the domain. It is more like model of the domain with description and constraints. This conceptualization knowledge is very important to have intelligent systems. The domain knowledge needs to be designed, modeled, stored, made available for processing and operations and communicated.
That is why ontology has been one of the hot concepts of web. Web being in need of solving interoperability demands a meaningful ontology realization. This can certainly be achieved using formalism and that has been the way!