Also called Neighbor or Index. A database such as Medline has been loaded into numerous search engines. Examples are Ovid, Silverplatter, Paper Chase, Dialog, and Medlars. Commands and conventions vary from engine to engine, but the structure of the database remains essentially unchanged in a reputable system. Set is a group of articles that results from a search command.
Operators can act on sets or words.
Subject Heading is a Thesaurus term. It is always part of a controlled vocabulary in which terms always have the same meaning and are applied uniformly. Thesaurus is a systematically compiled list, whose terms are used uniformly. Thesauri are subject or discipline specific and usually have a complex cross-referencing convention that refers users to related terms. The purpose is to show how terms are related to each other conceptually rather than the more typical alphabetical arrangement.