ontology

In AI an ontology is a form of {{semantic network}. In philisophy ontology is the general study of being, one aspect of which is classifying the kinds of things inthe world and deveopting formalisms to do this. It is this latter aspect which is used within computing and AI: ontologies as formal systems to describe physical and/or abstract things, ideas and relationships. Amongst other things, ontologies typically allow one to talk about (i) attributes of things (e.g. "Dumbo has big ears"), (ii) types/classes of things (e.g. "Dumbo is an elephant"), (iii) generalities about classes (e.g. "elephants have a trunk", (iv) sub-classes (e.g. "elephants re mamals"), rules (e.g. "X flies implies X has wings"), and (iv) exceptions to those rules. Note that there is a difference between the ontology fomalism/notation and the instances of that oformalism populated with a particular domian such as Disney film characters; however informally the word 'ontology' may refer to either depending oncontext.
Ontology formalisms and notations vary in their expressiveness, for example simple ontologies treat classes and objects completely separately and only have built in rules, more complex ontologies may allow classes to be treated in the same way as objects and allow rich rules. In general more complex ontologies can express more and allow more general reasonong, but that reasoning is harder. Only the simplest ontologies allow fully automated reasoning and verification.
Ontologies are used extensively in the Semantic Web building on RDF, RDFS (RDFS) and OWL (OWL). This has been used in fields, notably biology, to enable linked data resources that share common terminology, thus facilitating the reuse and recombination of data produced within the scietific community.
Fragment of RDF using RDFS:

Fragment of RDF using RDFS:

Defined on pages 394, 394, 434, 435

Used on pages 27, 32, 391, 393, 394, 395, 434, 435

Also known as ontologies