A query and rule language for deductive databases that syntactically is a subset of Prolog.


A vocabulary entailment of an RDF graph that respects the ‘meaning’ of data types.

Description Logic

A family of formal knowledge representation languages that are subsets of first order logic, but have more efficient decision problems.

Horn Logic

Broadly means a system of logic whose semantics can be captured by Horn clauses. A Horn clause has at most one positive literal and allows for an IF…THEN interpretation, hence the common term ‘Horn Rule’.

Knowledge Base

(In the Semantic Web sense) is a database of both assertions (ground statements) and an inference system for deducing further knowledge based on the structure of the data and a formal vocabulary.

Knowledge Representation

An area in artificial intelligence that is concerned with representing knowledge in a formal way such that it permits automated processing (reasoning).

Load Average

The load average represents the average system load over a period of time.


The process of inferring and storing (for later retrieval or use in query answering) every piece of information that can be deduced from a knowledge base’s asserted facts and vocabulary.

Named Graph

A group of statements identified by a URI. It allows a subset of statements in a repository to be manipulated or processed separately.


A shared conceptualisation of a domain, described using a formal (knowledge) representation language.


A family of W3C knowledge representation languages that can be used to create ontologies. See Web Ontology Language.


An entailment system built upon RDF Schema, see R-entailment.

Predicate Logic

Generic term for symbolic formal systems like first-order logic, second-order logic, etc. Its formulas may contain variables which can be quantified.

RDF Graph Model

The interpretation of a collection of RDF triples as a graph, where resources are nodes in the graph and predicates form the arcs between nodes. Therefore one statement leads to one arc between two nodes (subject and object).

RDF Schema

A vocabulary description language for RDF with formal semantics.


An element of the RDF model, which represents a thing that can be described, i.e., a unique name to identify an object or a concept.


A more general semantics layered on RDFS, where any set of rules (i.e., rules that extend or even modify RDFS) are permitted. Rules are of the form IF…THEN… and use RDF statement patterns in their premises and consequences, with variables allowed in any position.

Resource Description Framework (RDF)

A family of W3C specifications for modeling knowledge with a variety of syntaxes.

Semantic Repository

A semantic repository is a software component for storing and manipulating RDF data. It is made up of three distinct components:

  • An RDF database for storing, retrieving, updating and deleting RDF statements (triples);

  • An inference engine that uses rules to infer ‘new’ knowledge from explicit statements;

  • A powerful query engine for accessing the explicit and implicit knowledge.

Semantic Web

The concept of attaching machine understandable metadata to all information published on the internet, so that intelligent agents can consume, combine and process information in an automated fashion.


The most popular RDF query language.

Statement or Triple

A basic unit of information expression in RDF. A triple consists of subject-predicate-object.

Universal Resource Identifier (URI)

A string of characters used to (uniquely) identify a resource.