22. The method of claim 14 , wherein: determining that a term of the original query meets an inaccuracy criterion comprises determining that the term is an instance of a class of related terms, wherein the class of related terms are terms that are determined to be commonly confused terms; further comprising selecting each of the other terms that belong to the class as a candidate correction term; and generating one or more derivative queries from the original query comprises generating, for each of the candidate correction term, a derivative query that includes only the terms of the original query that are not the potentially inaccurate term and the candidate correction term.
14. A method implemented by a data processing apparatus, comprising: receiving an original query from a user device, the query including a plurality of terms; determining, by a data processing apparatus, that a term of the original query meets an inaccuracy criterion that is indicative of the term of the original query being a potentially inaccurate term in a context of the original query that is defined by the original query terms, wherein the determination is based at least in part on determining that a resource quality measure that is a measure of quality of resources identified as being responsive to the original query does not meet a resource quality measure threshold, and in response: identifying the term as a potentially inaccurate term; generating one or more derivative queries from the original query, each derivative query not including the potentially inaccurate term; for each of the one or more derivative queries, submitting the derivative query for a search of a resource corpus index of resources and receiving data identifying a respective set of resources that are determined to be responsive to the derivative query, wherein each resource is a web page resource hosted by a server and having a corresponding resource address; determining a corrected term based on the identified resources responsive to each of the one or more derivative queries, wherein the corrected term is determined based on respective sets of resources that are determined to be responsive to the one or more derivative queries and further determined independent of a set of resources identified as being responsive to the original query, and wherein the corrected term is a term that is different from the potentially inaccurate term; generating a corrected query that includes the terms of the original query that are not the potentially inaccurate term and that includes the corrected term substituted for the potentially inaccurate term; performing a search operation that uses the corrected query as input; and providing results of the search operation to the user device in response to the original query.