Source : Free On-Line Dictionary of Computing

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://www.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)