An index term, subject term, subject heading, or descriptor that captures the essence of the topic of a document.
Most Pay-Per-Click Search Engines allow bidding on Keywords to receive paid traffic. The bid is the amount, per click, that a marketer is willing to spend on a Keyword in order to have their advertisement displayed on a page where that Keyword was searched for.
Keywords make up a controlled vocabulary for use in bibliographic records, and in Search Engines. They are used as keywords to retrieve documents in an information system, for instance, a catalog or a search engine.
A popular form of keywords on the web are tags which are directly visible and can be assigned by non-experts also.
Keywords can consist of a word, phrase, or alphanumerical term. They are created by analyzing a document (in this case, a web page) either manually with subject indexing or automatically with automatic indexing or more sophisticated methods of keyword extraction. Keywords can either come from a controlled vocabulary or be freely assigned.
Keywords are stored in a search 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.
This article was based on the Wikipedia definition for Index term.