Big O: Ontology;   Little O: ontology
Philosophy: “a particular system of categories accounting for a
certain vision of the world” or domain of discourse, a
conceptualization  (Big O)
Artificial Intelligence: “an engineering product consisting of a
specific vocabulary used to describe a part of reality, plus a set
of explicit assumptions regarding the intended meaning of the
vocabulary words”, “a specification of a conceptualization”
(Little O)
Ontological Engineering: towards a formal, logical theory,
usually ‘concepts’ (i.e., the entities, usually classes
hierarchically structured in a special subsumption relation),
‘relations’, ‘properties’, ‘values’, ‘constraints’, ‘rules’,
‘instances’