1. A method implemented by a data processing apparatus, comprising: receiving an original query from a user device, the query including original query terms; receiving data identifying a set of web page resources that are determined to be responsive to the original query, each web page resource being a web page and identified by a corresponding URL address; determining, for the original query, a resource quality measure that is a measure of quality of the set of web page resources determined to be responsive to the original query; in response to determining that the resource quality measure does not meeting a resource quality measure threshold: determining, for each original query term of the query, a respective quality measure for the original query term, wherein each respective quality measure determined for an original query term is a quality measure that is different from the resource quality measure determined for the query; determining that an original query term of the original query term is a potentially inaccurate term based on the respective quality measure not meeting a respective quality measure threshold, and in response to determining that the original query term of the original query is a potentially inaccurate term: generating derivative queries from the original query, each derivative query not including the potentially inaccurate term; for each of the 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 that is different from the potentially inaccurate term based on the identified resources responsive to each of the one or more derivative queries, wherein the corrected term is determined independent of a set of resources identified as being responsive to the original query; generating a corrected query that that includes and 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.