An ontology is a formal and structured representation of concepts and relationships and attributes that define them. Ontologies can be formally described using their relations, attributes, restrictions and attributes.

Types of ontologies:

  • Upper Ontology: General language or common across all domains
  • Domain Ontology: represents concepts which belong to a specific domain of knowledge

Ontology components:

  • Concepts: A conceptualization of a cognitive entity. concepts of an ontology can be informally described using their synonyms, definition, examples, etc.
  • Classes(Set, collection, object type, or a kind of thing): A concept is defined as a single cognitive entity(anything that one can think of and describe) which describes parts of the world.
  • Individuals: instances or objects of a class.
  • Attributes: aspects, properties, features, characteristics, or parameters that objects (and classes) can have.
  • Relations: ways in which classes and individuals can be related to one another.
  • Function terms: complex structures formed from certain relations that can be used in place of an individual term in a statement.
  • Restrictions: formally stated descriptions of what must be true in order for some assertion to be accepted as input.
  • Rules: Statements in the form of an if-then (antecedent-consequent) sentence that describe the logical inferences that can be drawn from an assertion in a particular form. Used for computation. a rule could be any statement which says that a certain conclusion must be valid whenever a certain premise is satisfied.
  • Axioms: Each axiom is a set of simple statements, assertions, and rules in formal logical language which are used in reasoning and are asserted as priori knowledge.
  • Events: the changing of attributes or relations.
  • Actions: types of events.
  • Properties(and Roles) are a collection of relationships between instances

Considerations for creating an ontology:

  • Highly linked
  • Consistent
  • Intelligently designed


  • Ontology Population: creating ontologies from text by extracting relevant instances of information.
  • Domain Model: a specific domain which is used to represent the world or conceptualization of it.
  • Terminologies: domain specific multi-word terms or a group of terms that can form a valid concept.

TOO: study Knowledge Interchange Format (KIF)