The Free On-line Dictionary of Computing (30 December 2018):
The subfield of artificial intelligence concerned with
designing and using systems for storing knowledge - facts
and rules about some subject.
A body of formally represented knowledge is based on a
conceptualisation - an abstract view of the world that we
wish to represent. In order to manipulate this knowledge we
must specify how the abstract conceptualisation is represented
as a concrete data structure. An ontology is an explicit
specification of a conceptualisation.