&...; in RDF/XML — is this a standard RDF prefix expression?

Question

I downloaded WordNet2.0 fullset. In the RDF/XML document, wnfull.rdfs, there are elements like this the following that use, e.g., &wn20schema;AdverbSynset as attribute values (in places where URIs would be expected). I know how #, url/ and value: are commonly used, but what is this &...; notation?

<rdfs:Class rdf:about="&wn20schema;Synset" />  
<rdfs:Class rdf:about="&wn20schema;AdjectiveSynset" />  
<rdfs:Class rdf:about="&wn20schema;AdjectiveSatelliteSynset" />  
<rdfs:Class rdf:about="&wn20schema;AdverbSynset" />  

Answer 1

&wn20schema; is an entity reference.

The XML document should contain or point to its expansion, i.e., something like

<!ENTITY wn20schema "…">

&wn20schema; will be replaced with ….

References:

• XML 1.0: http://www.w3.org/TR/2008/REC-xml-20081126/#sec-physical-struct
  • See also examples.
• XML 1.1: http://www.w3.org/TR/2006/REC-xml11-20060816/#sec-physical-struct
  • See also examples.