A narrower term reference is made to a given subject heading. from the next broader heading so that terms are arranged in a hierarchy.

What is a narrower term?

Definition: An authorized, preferred heading that is narrower in definition and scope. Purpose: Supports the hierarchical relationship between or among headings in a controlled vocabulary.

What are broader terms?

A broader term indicates the more general class to which a term belongs in the controlled vocabulary; everything that is true of a term is also true of its broader term (e.g. Gossypium is broader than Gossypium hirsutum). ... This term may also be used instead of non-preferred term.