The term Ontology originally came from Greek language:

Ontos + Logia

(Gk) “being” + “the study of”

An ontology is a categorization of the world. One of the earliest Ontological concepts came from Plato: Platonic realism. It is the idea that everything that exists in reality has a corresponding conceptual truth that the extant thing is aspiring to be:

• There is a universal notion of ‘apple-ness’ that a real apple aspires to be

• This is one implementation of the idea that things in reality can have conceptual counterparts that are used to categorize them

This framework is mimicked in most Ontologies across the world.

Some of the inspiring examples of well-known ontologies include the Periodic Table of the Elements, Linnaean taxonomy or Latin naming system for animals.

The latter one is a useful example of an Ontology because we can see how Ontologies are frequently ordered hierarchically.

A hierarchy is a nice way to order concepts. Here we will re-use a simple explanation of ontology from the Practice Example 13.A , an exercise published on the AISpace.org, to provide an illustration of hierarchy use to build an ontology for pizzas.

Here is a reference describing the Web Ontology Language (OWL). Consider the following visualization of a OWL ontology:

Each word in the figure is a class and the arrows represent subclasses (e.g. Tomato is a subclass of Vegetable). Note that some of the class names in this ontology are purposefully misleading (for the purposes of this exercise).

These same concepts can be (and are) applied to other domains. For example, when organizations model their world, they care about how different objects (notably people) fit into a hierarchy.