18. The computer system of claim 15 , wherein replacing includes using a phrase similarity matrix, the phrase similarity matrix including, for each ambiguous term, one or more phrases associated with the ambiguous term.
15. A computer system for selecting sponsored content associated with content accessed over a computer network, comprising: a selection engine connected to the computer network, including a database including sponsored content associated with phrases; a plurality of user devices connected to the computer network; the selection engine being configured to receive one or more terms, to determine whether any of the terms is ambiguous such that the term, when combined with different other terms, produces different phrases which are associated with substantially different sets of items of sponsored content in the database, including to identify a first phrase that is a combination of an ambiguous term and one or more terms of the different other terms and identifying a second phrase that is a combination of the ambiguous term and one or more other terms of the different other terms and wherein the first phrase and the second phrase access different sets of items of the sponsored content, wherein determining a term is ambiguous includes identifying a plurality of phrases that contain or are related to the term; for each pair of phrases, determining a measure of similarity including determining a number of co-occurrences of the pair of phrases in a set of documents and a number of single occurrences of a phrase in the pair of phrases in the documents; determining that the pair of phrases are similar when the measure of similarity is above a threshold; and determining a term is ambiguous when two or more phrases of the plurality of phrases are dissimilar; and, for an ambiguous term, to replace the ambiguous term including selecting either the first or the second phrase both of which include the ambiguous term; the selection engine being further configured to select an item of sponsored content from the database using the selected one or the first or second phrase.