The following three terms: classifications, taxonomies and

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Classifications, Taxonomies, Ontologies, Thesauri

The following three terms: classifications, taxonomies and
ontologies are often confused.

This is caused by a trend to use the most fashionable of the three
terms “ontology”.

Classification: Definition

Webster definition:
systematic arrangement in groups or
categories according to established criteria.

An example of a classification would be the division of all plants in
the classes decorative and non

Taxonomy: Definition

Longman definition:
the process or a system of organizing things
into different groups that show their natural relationships,
especially plants or animals

Webster definition:
orderly classification of plants and
animals according to their presumed natural relationships

A taxonomy can described as (hierarchical) classification of items
created according to data internal criteria.

Ontology: Yet Another Definition

"The term

has many meanings and shades of meaning. In
the contest of this work*, we refer to what is sometimes called a
"structural" ontology

a machine readable set of definitions that
create a taxonomy of classes and subclasses and relationships
between them.“

Web Ontology (WebONT) Working Group Charter at

Classifications and Taxonomies: Comparison

Taxonomies are classifications that:

have hierarchical structure

list items that are organised according to their natural

Taxonomies and Ontologies

Most ontologies could be described as taxonomies + definitions +
various properties and relations (rather than just taxonomic, i.e.,
is_a, relations)

Thesauri and Ontologies

Nowadays, thesaurus most commenly means a listing of words
with similar, related, or opposite meanings.

"A formal definition of a thesaurus designed for indexing is:

a list of every important term (single
word or multi
word) in a given
domain of knowledge; and

a set of related terms for each term in the list.“*

"In Information Technology, a thesaurus represents a database or
list of semantically orthogonal topical search keys.

In the field of Artificial Intelligence, a thesaurus may sometimes be
referred to as an ontology.“*

Additional reading:


Animal Kingdom Taxonomy

Kingdom, class, order, genus, species

Gray wolves are dogs. Dogs are carnivore. Carnivores are
mammals. Mammals are animals.

National Center for Biotechnology Information (US)

Ontology Management and Maintenance Tools

Protégé is a free, open source ontology editor and knowledge
framework. Protégé ontologies can be exported into a variety of
formats including RDF(S), OWL, and XML Schema. For more

Protege Tutorial

Some tutorials:


Some Important Existing Ontologies


Suggested Upper Merged Ontology (SUMO):

WordNet Lexical reference system:

Gene Ontology:

Foundational, Core and Linguistic Ontologies:


Last but not Least