Social Tagging and Folksonomies

Some Definitions

This website is an attempt to consistently define key terms related to tagging and folksonomies. The definitions form the basis of the Modular Unified Tagging Ontology (MUTO) that is specified at:

A resource is anything that is uniquely addressable within a software system or computer network. In Web contexts, the term resource is broadly defined as "whatever might be identified by a URI" [RFC 3986, p. 5].

A tag is an arbitrary text label associated with a resource.
Automatic Tag
An automatic tag is a tag that is automatically assigned to a resource (e.g. by a tagging system).
Semantic Tag
A semantic tag is a tag with an explicitly defined meaning (e.g. expressed by a link to a well-defined entity).

Tagging (Def I)
Tagging is the annotation of resources by users with tags.
Social Tagging
Social tagging is tagging in an open online environment where the tags of the users are available to others.
Private Tagging
Private tagging is tagging that is only visible to its creator (unless the creator has not explicitly granted access to others).
Group Tagging
Group tagging is tagging performed by a group of users.
Automatic Tagging
Automatic tagging is tagging with automatic tags.
Semantic Tagging
Semantic tagging is tagging with semantic tags.

Tagging (Def II)
A single annotation resulting from tagging is also called tagging. The sum of these taggings is the folksonomy.
Axiom I
Each tagging links exactly one resource with one user account and one or more tags.
Axiom II
The number of tags per tagging is theoretically unlimited
(though it is practically limited by the constraints of the tagging system).
Axiom III
Each resource can be tagged at most once by each user account.
Axiom IV
Each tag can be assigned at most once to each resource by each user account.

"Folksonomy is the result of personal free tagging of information [...] for one's own retrieval. The tagging is done in a social environment (usually shared and open to others) [...] by the person consuming the information." [Van 2007]
Personomy is the sum of the taggings of a single user.

Tagging System
A tagging system is a software system that uses tagging for indexing
(either exclusively or in combination with other indexing methods).

Tag Cloud
A tag cloud displays "a certain number of most often used tags [...]. A tag's popularity is expressed by its font size (relative to the other tags) [...]. Sometimes, further visual properties, such as the font color, intensity, or weight, are manipulated [...]. Next to their visualization function, tag clouds are also navigation interfaces, as the tags are usually hyperlinks leading to a collection of items they are associated with." [LZT 2009]

Tagging Ontology

A powerful way to formally describe the main concepts and relationships of tagging is an ontology, i.e. "an explicit specification of a conceptualization" [Gru 1993]. One such ontology is the Modular Unified Tagging Ontology (MUTO). It unifies concepts from several tagging ontologies in one coherent schema. It supports different forms of tagging, such as normal, semantic, group, private, and automatic tagging, and is easily extensible. The specification of MUTO and a list of the unified tagging ontologies are available at


Creative Commons License
DEI logo