A compendium of synonyms and related terms.
A thesaurus organizes terms based on concepts and relationships between them. Relationships commonly expressed in a thesaurus include hierarchy, equivalence, and associative (or related). These relationships are generally represented by the notation BT (broader term), NT (narrower term), SY (synonym), and RT (associative or related). Associative relationships may be more granular in some schemes.
For example, the Unified Medical Language System (UMLS) from the National Library of Medicine has defined over 40 relationships across more than 80 vocabularies, many of which are associative in nature. Preferred terms for indexing and retrieval are identified. Entry terms (or non-preferred terms) point to the preferred terms that are to be used for each concept.