A frequently used word--usually an article, conjunction, or preposition with little semantic content--ignored when a keywords search is executed because it adds little value to the search statement and is not helpful for retrieval (examples: a, an, as, at, by, for, from, of, on, the, to). Some systems have a predetermined list of stopwords, which may be given in the help screen(s). In some systems, a stopword may be context-dependent, for example, the word "education" in a bibliographic database providing access to materials on education but not in a database indexing articles published in newspapers or general interest periodicals. Also spelled stop word.


From ODLIS, Online Dictionary for Libary and Information Science by Joan M. Reitz
http://lu.com/odlis/about.cfm