The Free On-line Dictionary of Computing (19 January 2023):

subject index

    An information resource that contains
   references to other resources, categorised by subject, usually
   in a hierarchy or ontology.

   Yahoo once maintained an Internet subject index.

   Search engines by comparison are based not on subject, but
   relevance, although this difference is often hidden from the
   user. Integration of knowledge representation into relevance
   ranking algorithms may reduce this distinction.

   (1997-04-09)