Ontologies

Fresh

Public ontologies converted to MD-LD format. This collection covers the most useful and meaningful ontologies maintained as core web standards.

Core Foundation Ontologies

MD-LD includes 5 foundational W3C ontologies by default. These define RDF as a functional system — every semantic statement in MD-LD is built upon these primitives.

RDF — Resource Description Framework

The fundamental grammar of linked data. RDF defines how statements are made: every fact is a triple of subject, predicate, object.

Includes: rdf:type, rdf:Property, rdf:Statement (reification), rdf:Bag, rdf:Seq, rdf:Alt, rdf:List, rdf:value, rdf:langString, rdf:HTML, rdf:XML, rdf:JSON

Use case: The meta-vocabulary for describing types, properties, and complex relationships. rdf:Statement is essential for adding context or metadata to statements themselves — recording who said something or when it was said.

RDFS — RDF Schema

The schema layer that sits atop RDF, providing vocabulary for describing classes, inheritance hierarchies, and domains/ranges of properties.

Includes: rdfs:Class, rdfs:Resource, rdfs:Datatype, rdfs:label, rdfs:comment, rdfs:seeAlso, rdfs:isDefinedBy, rdfs:subClassOf, rdfs:subPropertyOf, rdfs:domain, rdfs:range, rdfs:Container, rdfs:member

Use case: Organize concepts into taxonomies, annotate vocabulary with human-readable labels and descriptions, define which properties apply to which classes.

SHACL — Shapes Constraint Language

The validation framework for RDF graphs. SHACL allows you to define shapes — constraints that data must satisfy.

Includes: sh:NodeShape, sh:PropertyShape, sh:targetClass, sh:property, sh:path, sh:minCount, sh:maxCount, sh:datatype, sh:pattern, sh:message, sh:severity, sh:in, sh:node, sh:not

Use case: Make documentation self-validating. Define a shape for "User" records, enforce that usernames and emails are present, validate email patterns. SHACL turns descriptions into machine-checkable contracts.

PROV-O — W3C Provenance Ontology

The standard vocabulary for tracking who made what when and how.

Includes:

• prov:Entity — things with fixed aspects
• prov:Activity — processes that unfold over time
• prov:Agent — responsible parties (includes prov:Person, prov:Organization, prov:SoftwareAgent)
• prov:wasGeneratedBy, prov:used, prov:wasDerivedFrom, prov:wasAttributedTo, prov:wasAssociatedWith, prov:actedOnBehalfOf, prov:wasInformedBy
• prov:startedAtTime, prov:endedAtTime, prov:generatedAtTime

Use case: Trace the origin and evolution of data. Track which datasets fed into a pipeline, who ran the analysis, what agent was responsible. Essential for reproducibility, auditability, and attribution in AI/ML workflows.

XSD — XML Schema Definition Datatypes

The canonical vocabulary of literal types for RDF.

Includes: xsd:string, xsd:boolean, xsd:integer, xsd:decimal, xsd:double, xsd:float, xsd:dateTime, xsd:date, xsd:time, xsd:duration, xsd:anyURI, xsd:nonNegativeInteger, xsd:positiveInteger, xsd:dayTimeDuration, xsd:yearMonthDuration

Use case: Type safety and interoperability. A [2026-06-01] {ex:date ^^xsd:date} annotation is unambiguously a calendar date, not a string.

Quick Reference — Built-in Prefixes in MD-LD

These five are always available without declaration:

rdf:type          ← written as .Class
rdfs:label        ← written as label (bare)
rdfs:comment      ← written as comment (bare)
xsd:integer       ← [42] {my:count ^^xsd:integer}
prov:Person       ← .prov:Person
sh:NodeShape      ← .sh:NodeShape

Metadata and Discovery Ontologies

DCTERMS — Dublin Core Metadata Terms

50+ properties for describing any resource: dcterms:title, dcterms:creator, dcterms:date, dcterms:subject, dcterms:description, dcterms:license, dcterms:language, dcterms:format, dcterms:rights

Use case: Annotate any digital resource with metadata. The lingua franca of digital libraries, archives, and data portals.

DCAT — Data Catalog Vocabulary

W3C standard for publishing structured metadata about datasets and data services: dcat:Catalog, dcat:Dataset, dcat:Distribution, dcat:DataService

Use case: Build data portals that aggregate datasets from multiple agencies. Government open data platforms, research data repositories, corporate data governance.

FOAF — Friend of a Friend

Social ontology for describing people and organizations: foaf:Person, foaf:Organization, foaf:Group, foaf:Agent, foaf:name, foaf:email, foaf:homepage, foaf:knows, foaf:givenName, foaf:familyName

Use case: Model social networks, organizational structures, and contact information. Decentralized identity systems and machine-readable profiles.

VCard

Digital business cards and structured contact information: vcard:fn, vcard:given-name, vcard:family-name, vcard:email, vcard:tel, vcard:adr, vcard:url

Use case: Model rich contact profiles and directory data. The standard for sharing address books across applications.

Time and Events Ontologies

OWL-Time

Comprehensive formal ontology for temporal concepts: time:Instant, time:ProperInterval, time:Duration, time:before, time:after, time:inside, time:during, time:intervalIn

Use case: Model complex temporal requirements — project schedules, historical events, astronomical observations across different calendars.

Activity Streams 2.0

Vocabulary for modeling activities and event streams: as:Activity, as:Create, as:Update, as:Delete, as:Like, as:Follow

Use case: Build federated social media platforms, activity feeds, and real-time notification systems. Foundation of ActivityPub.

Knowledge Organization Vocabularies

SKOS — Simple Knowledge Organization System

Vocabulary for concept schemes: skos:Concept, skos:ConceptScheme, skos:broader, skos:narrower, skos:related, skos:exactMatch, skos:closeMatch, skos:prefLabel, skos:altLabel

Use case: Build controlled vocabularies — subject headings, medical terminology (SNOMED CT, ICD), domain-specific glossaries. Enables concept matching across languages.

Schema.org

Collaborative vocabulary for structured data on the web: schema:Person, schema:Organization, schema:Event, schema:Product, schema:Article, schema:Thing — over 800 types and 6000+ properties.

Use case: Markup web content for search engines and aggregators. Powers rich snippets, e-commerce, and knowledge graph construction.

Domain-Specific Ontologies

CIDOC CRM — Conceptual Reference Model

International standard for cultural heritage information. Models entities, physical objects, events, actors, temporal relationships, and provenance with museum-grade precision.

Use case: Document museum collections, archaeological artifacts, and historical records. Used by Europeana, the largest digital library of European cultural heritage.

SOSA — Sensor, Observation, Sample, and Actuator

Vocabulary for IoT, sensor networks, and scientific measurements: sosa:Observation, sosa:Sensor, sosa:Sample, sosa:FeatureOfInterest, sosa:madeBySensor, sosa:hasResult, sosa:observedProperty

Use case: Model weather stations, scientific instruments, laboratory experiments. Powers machine-to-machine data exchange in smart cities and precision agriculture.

QUDT — Quantities, Units, Dimensions and Types

Comprehensive vocabulary for scientific quantities and units: all SI units and common derived units with conversion factors. qudt:Quantity, qudt:Unit, qudt:QuantityKind

Use case: Annotate scientific and engineering data with precise units. Enables automatic unit conversion and dimensional analysis.

Web Standards Ontologies

OWL — Web Ontology Language

Formal knowledge representation with class constructors, cardinality constraints, property restrictions, and inverse properties. Enables logical reasoning.

Use case: Define formal ontologies when simple RDFS schemas are insufficient. Expert systems, semantic reasoning, and complex business rule engines.

Hydra — Hypermedia API Vocabulary

Vocabulary for describing hypermedia APIs in machine-readable form. Enables clients to discover and invoke operations dynamically.

Use case: Publish self-documenting APIs that clients can navigate without manual documentation.

Web Annotations

W3C standard for creating and exchanging annotations (comments, highlights, tags) on web resources: oa:Annotation with motivation types.

Use case: Build collaborative annotation platforms. Enable interoperable annotation exchange and open scholarly communication.