Synonyms and keywords
Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Synonyms are different words with identical or very similar meanings. Words that are synonyms are said to be synonymous, and the state of being a synonym is called synonymy.
The word comes from Ancient Greek syn ("with") and onoma ("name").
For example, the words car and automobile are synonyms. Similarly, if we talk about a long time or an extended time, long and extended become synonyms.
Keyword or index term or descriptor in Information Retrieval is a term that captures the essence of the topic of a document. It is used as keyword 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. Index terms can consist of a word, phrase, or alphanumerical term. They are created by analyzing the document either manually with subject indexing or automatically with automatic indexing or more sophisticated methods of keyword extraction. Index terms 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.