Library of Congress Subject Headings
A word or phrase assigned by the Library of Congress to a book or nonprint item to indicate what it is about. Multiple subject headings may be assigned to an item if necessary or desirable. It is also possible to append free-floating (subject) subdivisions to some subject headings, but only in accordance with strict rules laid down for their use, which are subject to revision over time.
The complete list of Library of Congress Subject Headings is published in a multi-volume set of large red books which is available in most public and academic libraries. Professional Catalogers in most libraries are also able to access the complete LCSH online through subscription to the Library of Congress web utility, Classification Web, which is usually bundled as an internal component of the Cataloger's Desktop subscription database.
The rules for free-floating subdivisions are published under separate cover and are normally not available for public inspection, as it is a non-circulating technical services cataloging tool, though some academic libraries situated in universities with an ALA-accredited LIS program granting MLS or MLIS degrees may circulate a copy for use by library school students taking cataloging courses.
The Library of Congress Subject Headings (LCSH) are a controlled vocabulary for use in subject cataloging and indexing. It was originally designed for the Library of Congress collection, but many other libraries have adopted the system as well, especially academic libraries.
An extremely well-written and useful book on the topic of LC subject headings is Lois Chan's Library of Congress subject headings : principles and application. 3rd ed. Englewood, Colo., 1995.