The Oxford dictionary describes an ontology as "a branch of metaphysics dealing with the nature of being".

Put simply: An ontology is a collection of concepts or terms of a given domain of knowledge/area of existence and semantics describing these concepts and the relationships between the concepts. It is like a more sophisticated dictionary-cum-thesaurus-cum-database.

The term ontology/ontologos is in fact a neologism meaning “to reason about being.” It is constructed from the prefix onto- that means “being” or “existence,” and the base logos that means “to reason,” referring to the science that deals with the principles and criteria of validity of inference and demonstration – the science of the formal principles of reasoning.

There are various levels of description starting from simple categorically organised thesauri or dictionaries of terms to more complex descriptions with automated reasoning. Examples are listed below in order of complexity: