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, 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.