This article discusses the different meanings of ‘keyword’ and related terms such as ‘keyphrase', ‘descriptor’, ‘index term’, ‘subject heading’, ‘tag’ and ‘n-gram’ and suggests definitions of each of these terms. It further illustrates a classification of keywords, based on how they are produced or who is the actor generating them and present comparison between author-assigned keywords, indexer-assigned keywords and reader-assigned keywords as well as the automatic generation of keywords. The article also considers the functions of keywords including the use of keywords for generating bibliographic indexes. The theoretical view informing the article is that the assignment of a keyword to a text, picture or other document involves an interpretation of the document and an evaluation of the document’s potentials for users. This perspective is important for both manually assigned keywords and for automated generation and is opposed to a strong tendency to consider a set of keywords as ideally presenting one best representation of a document for all requests.