13. The method of claim 8 , further comprising: receiving an initial search query including one or more terms; determining that a first term in the search query occurs in a synonym pair of the one or more synonym pairs; in response to determining that the first term in the search query occurs in a synonym pair of the one or more synonym pairs, generating an expanded search query that includes a second term of the synonym pair; obtaining search results using the expanded search query to search a collection of documents; and providing the search results in response to the initial search query.
8. A computer-implemented method comprising: identifying a plurality of queries in a query log, and for each query, one or more search results associated with the query, wherein each search result identifies a corresponding resource and comprises a search result snippet that includes text from the corresponding resource identified by the search result; generating a plurality of query-snippet pairs, wherein each pair associates a respective query of the plurality of queries with a particular search result snippet from one of the search results associated with the query in the query log; training a statistical machine translation model using the plurality of query-snippet pairs, including using each query of each query-snippet pair as a source language input, and a corresponding snippet of the query-snippet pair as a target language input, wherein each query and the search result snippet of each query-snippet pair are in the same natural language; translating, using the statistical machine translation model trained on the plurality of query-snippet pairs and search result snippet pairs, a phrase into one or more corresponding translated phrases; and determining one or more synonym pairs including comparing the phrase with the one or more corresponding translated phrases.