One of the reasons for disagreement is the broad definition that has been proposed to group all current ontologies. Wikipedia defines ontology as a “Formal representation of a set of concepts within a domain and the relationships between those concepts”. This definition might well describe any other Knowledge Organization System or terminological resource. Wikipedia adds that ontologies are “used to reason about the properties of that domain, and may be used to define the domain”.

The best known definition was proposed by Gruber: “a formal, explicit specification of a shared conceptualization.” (1991).

1. Modeling techniques

The two most commonly used techniques are:

• First-order logic
• Description logic

2. Ontology elements

Depending on the technique that has been used, the vocabulary for designing some elements might differ.

• Classes: a set of similar objects. These sets represent the main concepts of the domain and the concepts are often arranged in a hierarchical way. Classes might have attributes and functions and can be linked to another class by relations.
• Relations: Relationships linking classes and individuals
• Attributes: properties that classes and instances can have.
• Functions: class and instance methods or actions
• Individuals: instances or objects of a class.

To achieve inference, the existence of true assertions has to be established: these assertions are then used to express restrictions, rules and axioms.

Finally, events are a way to represent how the values of attributes and their relationships might change.

First order logic usually names these elements as: classes, relations, attributes (slots), functions, instances, and axioms.

Description Logic has the following elements: concepts (equivalent to classes); roles (equivalent to relations and properties of concepts); and Individuals (equivalent to the instances of concepts and their properties).

• Clarity: objective definitions, formalized with axioms, and complete (necessary and sufficient conditions).
• Minimal Encoding Bias
• Extendibility
• Minimal ontological commitments

• Representing disjoints and exhaustive knowledge
• Minimizing distance between siblings
• Standardizing names in a clear form

4. Types of Ontologies

There are different types of ontologies:

• Upper (~top or foundational ): describe very general concepts that are common to all the ontologies. Other ontologies can be aligned with these concepts by their root term. Examples are DOLCE, Proton, SUMO, and CYC.
• Task: describes the vocabulary related to some generic task or activity.
• Domain: concepts of a domain and their relationships.
• Common (~generic ontologies): common knowledge reusable in different domains. Examples are ontologies about time or space.
• Knowledge Representation: primitives to express knowledge in a formalized way.
• Application: an ontology adapted to a specific application.

Regarding the usability and reusability, most abstract ontologies are highly reusable (e.g. Knowledge Representation Ontologies and Upper Ontologies) but their usability is poor. Application and Domain Ontologies have a low reusability but a high level of usability.

5. Languages

Modeling techniques use  different languages. As an example, one of the languages related to first order logic is KIF. Whereas OWL is the language widely used to represent ontologies on the Web. OWL serialization is based on RDF/RDFS.