The Free On-line Dictionary of Computing (30 December 2018):

subject index

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

   Yahoo is the most popular Internet subject index.  Like
   most other subject indices

(http://yahoo.com/Computers_and_Internet/Internet/World_Wide_Web/Searching_the_Web/Web_Directories/),
   Yahoo is arranged ontologically.

   Subject indices are not to be confused with search engines,
   which are based not on subject, but instead on relevance,
   although (1) this difference is often (possibly rightly)
   hidden from the unsophisticated user, and (2) future
   integration of knowledge representation into relevance
   ranking algorithms will make this a hazy distinction.

   (1997-04-09)