7. A computing system comprising: one or more computers; and a computer-readable medium having instructions stored thereon that, when executed by the one or more computers, cause performance of operations comprising: identifying, by the computing system, that a user input at a first computing device selected, from among a set of search results provided to the first computing device responsive to a first query, a search result that references a first electronic document; generating, by the computing system and in response to identifying that the user input at the first computing device selected the search result that references the first electronic document, an association between the first electronic document and one or more terms derived from the first query; after generating the association between the first electronic document and the one or more terms derived from the first query: receiving a second query specified by a user of a second computing device; determining a relevance of the first electronic document to the second query based at least in part on a level of similarity between (i) the one or more terms derived from the first query and associated with the first electronic document and (ii) one or more second terms derived from the second query; generating a second set of search results responsive to the second query, including selecting or ranking the first electronic document represented in the second set of search results based on the determined relevance; and transmitting the second set of search results to the second computing device.
9. The computing system of claim 7 , wherein the operations further comprise: identifying, by the computing system, that a second user input selected, from among a third set of search results provided to a computing device responsive to a third query, a particular search result that references the first electronic document; and generating, by the computing system and in response to identifying that the second user input selected the particular search result that references the first electronic document, a second association between the first electronic document and the one or more terms derived from the third query.