RDF Terminology and Concepts

Thu 16 August, 2001
Martyn Horner
25 July 2001
Graham Klyne
Graham Klyne
Brian McBride
Bill dehOra
Dan Brickley
Pat Hayes

This is currently a live working document, being a collection of suggestions from participants in the W3C RDF Interest and RDF Core Working Groups.

MH: I've tentatively re-arranged the material and added some background (without removing much) with two objectives:

  1. to remove forward references so that a new reader can work through this glossary to build an understanding of the basic terms, and
  2. to separate model and serialization terms (in the spirit of the face-to-face meeting's feelings about the model being the core concept) and the definitions of the RDF model and its associations with the world (the entity, resource, URI thread).

I've inserted some background material into the text in a hope that this will allow mixing of text in situ and avoid the sudden introduction of incompatible viewpoints from out of the blue.

While not wishing to produce a redundant document, I feel that a `tutorial' glossary is useful to help a reader form an internally consistent language before receiving the precise definitions of the specs. It is therefore important not to give the user redolent but false images which may cause him/her to resist the finer descriptions when they are encountered. Definitions here should be `natural' and easily assimilated.



The RDF Model and Syntax Specification.

Entities, etc

Data describing Web resources [RDFM&S].
Web Resource
Anything that is identified by a URI
RFC2396 (in context of defining URI) defines resource

A resource can be anything that has identity. Familiar examples include an electronic document, an image, a service (e.g., "today's weather report for Los Angeles"), and a collection of other resources. Not all resources are network "retrievable"; e.g., human beings, corporations, and bound books in a library can also be considered resources. The resource is the conceptual mapping to an entity or set of entities, not necessarily the entity which corresponds to that mapping at any particular instance in time. Thus, a resource can remain constant even when its content---the entities to which it currently corresponds---changes over time, provided that the conceptual mapping is not changed in the process

Anything which exists or has existed.
Note that RFC2396 uses this term in a more restricted sense, to mean some data represents some aspect of a Web Resource.
RDF Resource
[See RDF M&S section 5] Note that an RDF resource is not necessarily a web resource, though any web resource can be an RDF resource.

Consider: http://foo.com/#a and http://foo.com/#b may name distinct RDF resources, but if used to access web resources they both refer to the common web resource http://foo.com/

This distinction between "Web resource" and "RDF Resource" is not a desired outcome, but an interpretation of different uses of the term "resource" in different documents.

May refer to an RDF resource or a Web Resource.
Some resources may be both. In discussion of RDF, this term is often used to mean RDF Resource.
RDF Resource Identifier, Resource Identifier
A URI plus optional anchor ID. [RDFM&S] RDF Resource Identifiers are understood to name RDF Resources.
The entity or concept that an RDF Resource describes. [RDFM&S]

RDF Model

RDF Statement, Statement
RDFM&S section 5: There is a set called Statements, each element of which is a triple of the form {pred, sub, obj} Where pred is a property (member of Properties), sub is a resource (member of Resources), and obj is either a resource or a literal (member of Literals).
A statement expressed as a collection of three items: subject, predicate and object.
[See RDFM&S section 5]. (This term has caused some confusion, since it has a quite specific meaning to logicians, which is not the same as some would regard as its "natural" meaning.) This term is used in three distinct ways:
  1. The RDF Model, meaning the underlying structure and interpretation of RDF data (see Proposed model theory by Pat Hayes)
  2. An RDF Model, meaning an instance of a collection of RDF statements
  3. Logical Model, being a formal logicians' term with quite specific meaning. (see http://www-rci.rutgers.edu/~cfs/305_html/Deduction/FormalSystemDefs.html).

RDF Graph
A set of RDF Statements.

RDF serializations

An expression of an RDF
RDF Description
[See RDFM&S] Construct containing representations of a number of RDF statements about a specific RDF resource, and possibly some additional statements.
I don't find this very helpful as an aid to understanding (`possibly') [MH]
Distributive Referent
A Referent that describes each of the Resources in a container, not including the container. The Referent is said to be made on the container. [RDFM&S]
Description [of]
(As opposed to RDF Description) Language or data structure providing information about some entity or concept.
Stand for
The use of one entity or concept in a description to refer to some other entity or concept. For example, "X stands for Y in Z" meaning that occurrences of "X" in "Z" are to be understood as references to "Y".


Reification of an RDF Graph
A (bag/collection?) containing the reifications of the statements in an RDF Graph
Reified Graph
The (bag/collection?) in the reification of an RDF graph.
Higher Order Statement
A Statement whose Referent is another Statement.[RDFM&S]
Higher Order Context
A Context whose Referent is another Context.
Denote. (v) The fundamental semantic relationship between the syntactic and semantic domains; the relationship between an expression and the entity it is interpreted to mean or refer to, expressed (somewhat misleadingly) as an activity of the expression oriented towards the thing. For example, a person can be said be denoted by their name.

Hence, denotation (n), the thing or things denoted by a name or expression.

Exactly what counts as a suitable denotation for some kinds of expression has been the subject of much debate, eg assertional sentences may be said to denote truth-values, or propositions, or functions from possible worlds, etc.. Typically, a given semantic theory for a human language takes a particular stance on such issues, providing a precise analysis of some range of intuitive meanings while excluding others from consideration. For formal languages, the range of denotations is usually specified mathematically. In formal semantics, an interpretation of a language is specified by rules which determine the denotations of complex expressions in terms of the denotations of their subexpressions, often called a truth-recursion.

Note. The relationship between a sign and what it denotes - the denotation relationship - is not considered to have any particular causal or physical significance, in general. Philosophers have noted that if denotation were a physical relationship then it would travel faster than light every time an astronomer mentions a star. Similarly, there is no way, in general, to compute the denotation of a name from the name itself. Both of these observations follow from the fact that the denotation of any expression is only defined relative to an interpretation of the language or notation in which the expression occurs.

Reification (of a statement)
[See RDFM&S section 5] A resource that stands for the statement together with the four statements that describe the statement. More than one reification may exist for a given statement.
(There is some debate whether multiple reifications of a statement are necessarily equivalent.)
Reified Statement
[See RDFM&S section 5] A resource that stands for a statement in a Reification. This resource has four properties describing the statement, and maybe others.
Reification Quad
The four statements that make up a Reification.
A data structure (abstract or concrete) that captures some essential properties of some entity or concept.
Representing [x]
Being a representation of [x] (see above)
An environment within which some statements are taken to be true.
A reference to a statement without necessarily making any assertion about its truth or falsity.
The expression of an RDF statement [or set of statements] in some context of discourse that is taken to be an assertion of the truth of the statement[s] in that context.



John McCarthy at http://www-formal.stanford.edu/jmc/generality/node6.html:

Reasoning about knowledge, belief or goals requires extensions of the domain of objects reasoned about. For example, a program that does backward chaining on goals used them directly as sentences, e.g. on(Block1,Block2), i.e. the symbol on is used as a predicate constant of the language. However, a program that wants to say directly that on(Block1,Block2) should be postponed until on(Block2,Block3) has been achieved, needs a sentence like precedes(on(Block2,Block3),on(Block1,Block2)), and if this is to be a sentence of first-order logic, then the symbol on must be taken as a function symbol, and on(Block1,Block2) regarded as an object in the first order language.

This process of making objects out of sentences and other entities is called reification. It is necessary for expressive power but again leads to complications in reasoning. It is discussed in (McCarthy 1979).

Tim Berners-Lee has acknowledged that the last paragraph is the intent of reification in RDF (http://lists.w3.org/Archives/Public/www-rdf-logic/2001Jan/0048.html).


RDF issues

Dan Brickley maintains a page of RDF specification issues at http://www.w3.org/2000/03/rdf-tracking/.

Brian McBride also has a list of issues at http://www-uk.hpl.hp.com/people/bwm/rdf/issues.htm. Against each listed issue are pointers to messages on the RDF IG mailing list, and/or other relevant threads of commentary.

Additional resources

Some related resources / context:

http://www.w3.org/Help/siteindex W3C site index / technology keywords

http://www.w3.org/People/Berners-Lee/Weaving/glossary.html Glossary from 'Weaving the Web'.

http://www.w3.org/WCA/ Web Characterisation Initiative (historical interest)

http://www.w3.org/1999/05/WCA-terms/ Web Characterization Terminology & Definitions Sheet W3C Working Draft 24-May-1999 HTTP-NG Activity Statement (historical interest)


http://www.w3.org/TR/WD-HTTP-NG-interfaces/ HTTP-NG Web Interfaces (an attempt to formalise our a notion of URIs, resources etc in terms of a distributed object type hierarchy).

http://www.w3.org/Addressing/ Naming and Addressing: URIs, URLs, ...

http://www.ietf.org/rfc/rfc2396.txt -- URIs

http://www.w3.org/Protocols/rfc2616/rfc2616.html -- for HTTP 1.1's notion of URI, 'resource', entity etc...


Insightful comments were provided by Pierre-Antoine Champin.