• OPAC (Online Public Access Catalog)A library or other catalog of content objects that is accessible online. The catalog may or may not be accessible to the public, but it is still called an OPAC.
  • organizational warrantJustification for the representation of a concept in an indexing language or for the selection of a preferred term due to characteristics and context of the organization. See also literary warrant and user warrant.
  • orphan termA term that has no associative or hierarchical relationship to any other term in a controlled vocabulary.
  • orthographyThe art of writing words with the proper letters according to standard usage.