A word, a phrase or a group of words, possibly combined with other syntax used to pass instructions to a search engine or a directory in order to locate web pages.
For details of which queries are being used, visit the Overture.com Search Inventory page. To "spy" on queries as they're entered, look at the Metaspy page.  A free program called Word Market will collect search terms from the search engines, and is available at http://www.softwaresolutions.net/free.htm.