An ontology library (currently also called distributed ontology on Ontohub) is a collection of ontologies and mappings (these are called the children of the ontology library). Ontology libraries can be written in DOL and uploaded to Ontohub.
The URI of a child of an ontology library consists of the URI of the library, two slashes, and the name of the ontology. E.g., http://ontohub.org/myRepository/myOntologyLibrary//myOntology
Individual ontologies are sometimes called single ontologies in order to distinguish them from ontology libraries.