A construct placed in the HTML header of a web page, providing information which is not visible to browsers. The most common meta tags (and those most relevant to search engines) are KEYWORDS and DESCRIPTION.
The KEYWORDS tag allows the author to emphasise the importance of certain words and phrases used within the page. Some search engines will respond to this information - others will ignore it. Don't use quotes around the keywords or keyphrases.
The DESCRIPTION tag allows the author to control the text of the summary displayed when the page appears in the results of a search. Again, some search engines will ignore this information.
The HTTP-EQUIV meta tag is used to issue HTTP commands, and is frequently used with the REFRESH tag to refresh page content after a given number of seconds. Gateway pages sometimes use this technique to force browsers to a different page or site. Most search engines are wise to this, and will index the final page and/or reduce the ranking. Infoseek has a strong policy against this technique, and they might penalize your site, or even ban it.
Other common meta tags are GENERATOR (usually advertising the software used to generate the page) and AUTHOR (used to credit the author of the page, and often containing e-mail address, homepage URL and other information).

This definition is in context to Search Engine Optimization. See more contextual defintions for Meta Tag.