关键字（Keyword），特指单个媒体在制作使用索引时，所用到的词汇。是图书馆学中的词汇，关键字搜索是网络搜索索引主要方法之一。

例如一本书的书名或部分书名、副标题、作者名都可以作为本书的关键词用于检索。现在大部分的图书及网上检索都是用关键字检索的形式。

相对于关键字，在检索中还有导出词的应用。

An index term, subject term, subject heading, or descriptor, in information retrieval, is a term that captures the essence of the topic of a document. Index terms make up a controlled vocabulary for use in bibliographic records. They are an integral part of bibliographic control, which is the function by which libraries collect, organize and disseminate documents. 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. 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.