Ontology is the philosophical study of the nature of being, becoming, existence or reality as well as the basic categories of being and their relations - wikipedia
In computer science and information science, an ontology is a formal naming and definition of the types, properties, and interrelationships of the entities that really or fundamentally exist for a particular domain of discourse. It is thus a practical application of philosophical ontology, with a taxonomy - wikipedia
The fields of artificial intelligence, the Semantic Web, systems engineering, software engineering, biomedical informatics, library science, enterprise bookmarking, and information architecture all create ontologies to limit complexity and to organize information. The ontology can then be applied to problem solving.
A survey of ontology visualization techniques is presented by Katifori et al. An evaluation of two most established ontology visualization techniques: indented tree and graph is discussed in. A visual language for ontologies represented in OWL (Web Ontology Language) is specified by the Visual Notation for OWL Ontologies (VOWL) - wikipedia
Head from a herm discovered at an excavation in Velia in 1966 - wikimedia
Werner Ceusters has noted the confusion caused by the significant differences in the meaning of word ontology when used by philosophy compared with the use of the word ontology in computer science, and advocates for greater precision in use of the word ontology so that members of the various disciplines using various definitions of the word ontology can communicate.
He writes 'before one is able to answer the question 'what is an ontology?', one must provide first an answer to the question 'what does the word ontology mean?' - wikipedia
It's also not clear how ontology fits with Schema on Read (NoSQL) databases.