A Related Term is related to the Preferred Term in a Thesaurus or Controlled Vocabulary by an Association.

Since this relationship is quite arbitrary, it is like the figure of speech Metaphor?, where some human being sees an otherwise arbitrary relationship.

A Broader Term or Narrower Term has a relationship to the Preferred Term of Hierarchy?, containers and contained. The corresponding figure of speech is called Metonym?.

A Variant Term or Used For term has the relationship of Equivalence or figure of speech Synonym?.

Arbitrary relationships between resources (at URLs) is the core concept behind the Semantic Web.