A keyword (also index term or descriptor) in Information Retrieval is a term that captures the essence of the topic of a document or a search query. When a descriptor is descriptive, it can be an effective search parameter. It is used to retrieve documents in an information system, for instance a catalog or a search engine. It can consist of a word, phrase, or alphanumerical term. Keywords are created by analyzing the document either manually with subject indexing or automatically with fulltext indexing or more sophisticated methods of keyword extraction. Keywords are stored in an Index. Common words like articles (a, an, the) and conjunctions (and, or, but) are not treated as keywords because it is inefficient to do so. Almost every English-language site on the Internet has the article "the", and so it makes no sense to search for it. The most popular search engine, Google removed Stop words such as "the" and "a" from its indexes for several years, but then re-introduced them, making certain types of precise search possible again. The term "descriptor" was coined by Calvin Mooers in 1948. A popular form of keywords on the web are tags which are directly visible and can be assigned by non-experts also.
