18. The medium of claim 13 , wherein the generated score is based on an extent to which users switched between the first and second terms when entering search queries.
13. A non-transitory computer-readable medium storing software comprising instructions executable by one or more computers which, upon such execution, cause the one or more computers to perform operations comprising: receiving a query comprising a first term; identifying one or more synonyms for the first term, wherein identifying the one or more synonyms comprises: determining that the first term and a second term satisfy predetermined criteria associated with a particular class of related terms, in response to determining that the first term and the second term satisfy the predetermined criteria, selecting a threshold that is different from a default threshold, wherein the default threshold is a threshold that is used to evaluate terms that do not satisfy the predetermined criteria associated with the particular class of related terms, generating a score for the first term and the second term by evaluating a relationship between the first term and the second term, and determining that the second term is a synonym of the first term based on applying the generated score to the selected threshold; generating an altered query using the second term; and processing the altered query to produce search results.