13. A computer-readable medium encoded with a computer program, the program comprising instructions to perform operations, the operations comprising: receiving data identifying a knowledge item; retrieving, from a database, document-based information to be used in selecting a meaning for the knowledge item, where the document-based information comprises one or more documents related to the knowledge item; determining one or more document term meanings of the document-based information, the determining comprising, for each document 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 of the terms, and then determining one or more document term meanings of the document from the one or more meanings for terms in the document; determining a plurality of knowledge item term meanings of the knowledge item by selecting a plurality of meanings from meanings associated with one or more terms of the knowledge item in the computer-readable data collection; determining a strength of relationship between each knowledge item term meaning and each document term meaning of the document-based information, and determining a respective selection probability for each knowledge item term meaning from the strengths; and designating knowledge item term meaning from the plurality of knowledge item term meanings as a meaning of the knowledge item according to the respective selection probability associated with each knowledge item term meaning; and storing the designated meaning of the knowledge item.
13. A computer-readable medium encoded with a computer program, the program comprising instructions to perform operations, the operations comprising: receiving data identifying a knowledge item; retrieving, from a database, document-based information to be used in selecting a meaning for the knowledge item, where the document-based information comprises one or more documents related to the knowledge item; determining one or more document term meanings of the document-based information, the determining comprising, for each document 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 of the terms, and then determining one or more document term meanings of the document from the one or more meanings for terms in the document; determining a plurality of knowledge item term meanings of the knowledge item by selecting a plurality of meanings from meanings associated with one or more terms of the knowledge item in the computer-readable data collection; determining a strength of relationship between each knowledge item term meaning and each document term meaning of the document-based information, and determining a respective selection probability for each knowledge item term meaning from the strengths; and designating knowledge item term meaning from the plurality of knowledge item term meanings as a meaning of the knowledge item according to the respective selection probability associated with each knowledge item term meaning; and storing the designated meaning of the knowledge item. 24. The computer-readable medium of claim 13 , further operable to cause processors to perform operations comprising: determining a semantic sub-space defined by a radius of semantic distance from the knowledge item meaning; identifying an advertisement having an advertisement meaning that falls within the semantic sub-space; and presenting the advertisement.