1. ontology – (computer science) a rigorous and exhaustive organization of some knowledge domain that is usually hierarchical and contains all the relevant entities and their relations organization, arrangement, organisation, system – an organized structure for arranging or classifying; “he changed the arrangement of the topics”; “the facts were familiar but it was in the organization of them that he was original”; “he tried to understand their system of classification”
computer science, computing – the branch of engineering science that studies (with the aid of computers) computable processes and structures
2. ontology – the metaphysical study of the nature of being and existence. Related to metaphysics – the philosophical study of being and knowing.