25. A computer-implemented method for outputting advertisements related to web page content, comprising: receiving a keyword; retrieving, from a database, document-based information to be used in selecting a meaning for the keyword; determining, in a computer system comprising one or more servers, one or more document term meanings of the document-based information, the determining comprising, for one or more documents in the document-based information; selecting one or more meanings for terms in the document from meanings associated with the terms in a computer-readable data collection that associates at least one meaning with each term, and then determining one or more document term meanings of the document from the one or more meanings for terms in the document; determining, in the computer system, a plurality of candidate keyword meanings of the keyword by selecting a plurality of meanings from meanings associated with the keyword in the computer-readable data collection; determining, in the computer system, a strength of relationship between each candidate keyword meaning and each document term meaning of the document-based information, and determining a respective selection probability for each candidate keyword meaning from the strengths; selecting a keyword meaning from the plurality of candidate keyword meanings according to the respective selection probability of each candidate keyword meaning; matching the keyword to web page content associated with a web page; determining a semantic sub-space defined by a radius of semantic distance from the keyword meaning, identifying an advertisement having an advertisement meaning that falls within the semantic sub-space, and matching the keyword to the advertisement; associating, in the computer system, the advertisement with the web page content; and outputting the advertisement when the web page is displayed.
29. The method of claim 25 , wherein the document-based information comprises search results associated with the keyword.