8. A system comprising one or more processors and one or more storage devices storing instructions that are operable, when executed by the one or more processors, to cause the one or more computers to perform operations comprising: selecting a hypernym from a hypernym repository; identifying a set of queries, from a query repository, that each include one or more terms that are hyponyms of the selected hypernym; determining a measure of specificity for the selected hypernym based at least on a comparison between (i) a probability distribution for co-occurrence of the one or more terms in the set of queries that are hyponyms of the selected hypernym, and (ii) a background probability distribution for the occurrence of the terms in the set of queries; determining whether the measure of specificity for the selected hypernym satisfies a threshold; and based on determining that the measure of specificity for the selected hypernym satisfies the threshold, determining whether to revise a subsequently received search query that includes one hyponym of the selected hypernym, to include another hyponym of the selected hypernym.
8. A system comprising one or more processors and one or more storage devices storing instructions that are operable, when executed by the one or more processors, to cause the one or more computers to perform operations comprising: selecting a hypernym from a hypernym repository; identifying a set of queries, from a query repository, that each include one or more terms that are hyponyms of the selected hypernym; determining a measure of specificity for the selected hypernym based at least on a comparison between (i) a probability distribution for co-occurrence of the one or more terms in the set of queries that are hyponyms of the selected hypernym, and (ii) a background probability distribution for the occurrence of the terms in the set of queries; determining whether the measure of specificity for the selected hypernym satisfies a threshold; and based on determining that the measure of specificity for the selected hypernym satisfies the threshold, determining whether to revise a subsequently received search query that includes one hyponym of the selected hypernym, to include another hyponym of the selected hypernym. 11. The system of claim 8 , wherein the terms comprise individual words.