Taxonomy, Thesaurus, Ontology... What the # are they talking about?

These terms are very often used interchangeably… and cause a lot confusion doing so! These ‘things’ happen to be our expertise at TenForce: we develop them and make systems that use them. And pragmatic as we are we usually don’t bother too much about the right name. ‘Cause what’s in a name?  That which we call a taxonomy by any other name would work as well. But we don’t want you to be confused either, so let’s go for a short ride through the land of Knowledge Organization Systems.

What do they have in common?

Taxonomies, thesauri and ontologies are systems that help you organize your knowledge.

Think of a library: the knowledge is kept in books and placed on shelves in an alphabetical order. A very practical system! We all know how to find a book when we know the title or the author.

This is a simple example of a KOS – this is what geeks say when they talk about a Knowledge Organization System – and yet learns us some basic principles. The system of how we structure our ‘knowledge’ – or books - should be unambiguous, so that you look for a book at the same place where the librarian put it.

… and how are they different?

The level of complexity is different.

• A taxonomy is a hierarchical structure in which every category has a name.
• A thesaurus is a hierarchical structure – like a taxonomy – but includes more information than just a name: synonyms, an explanation of the category, related categories, etc.
• An ontology is typically not restricted to hierarchical relations between categories, but can express how things are related to each other in other ways as well.

An example, please!

In a taxonomy you could find that this ‘rosa canina’ is one of the subcategories of ‘rosaceae’, which in its turn belongs to the category of plants. A thesaurus could tell you that ‘dog rose’ is a more popular name for the same rose. But only an ontology can express that a rosa canina a.k.a. the dog rose “smells sweet” and can be “pink or white”. 

Little trick to remember the difference between ‘taxonomy’ and ‘thesaurus’?

“Thesaurus” means “treasure” in Greek (note the resemblance!) and as such is more rich than a simple taxonomy (literally “arrangement method”).

What's in a name? That which we call a rose by any other name would smell as sweet.

There is a substantial difference however between ontologies on the one hand and taxonomies and thesauri on the other. Taxonomies and thesauri are ‘controlled vocabularies’, they intend to control the names you give to something. Ontologies do more: they capture knowledge in a model by looking at the properties of things, and how they are related.

• In our training ‘Introduction to Semantic Technology’ we explain deeper the subtle differences between these systems, and more importantly, how they are used in IT systems. Find out more at the training page. 
• Find out what we do with taxonomies and thesauri.