Ontology is a branch of philosophy. It is concerned with questions including how are objects grouped into categories?
An ontology is a collection of sets of objects and properties (or maybe: a collection of sets of points in thingspace). An agent's ontology determines the abstractions it makes.
For example, "chairs"_Alice is in Alice's ontology; it is (or points to) a set of (possible-)objects (namely what she considers chairs) that she bundles together. "Chairs"_Bob is in Bob's ontology, and it is a very similar set of objects (what he considers chairs). This overlap makes it easy for Alice to communicate with Bob and predict how he will make sense of the world....