42. A computer-readable medium encoded with a computer program, the program comprising instructions to perform operations, the operations comprising: receiving a keyword; retrieving, from a database, document-based information to be used in selecting a meaning for the keyword, where the database associates each of a plurality of keywords with information related to the keyword, and the document-based information is related to the keyword; determining 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 includes terms and 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 a 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 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.
46. The computer readable medium of claim 42 , wherein the document-based information further comprises search results associated with the keyword.