7. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: identifying a category from a knowledge base, the category including a plurality of entities; identifying a set of queries including terms related to the entities included in the category from a query stream; calculating a specificity value for the category, based at least on the entities included in the category and the set of queries, comprising: determining a probability distribution for co-occurrence of the terms in the set of queries with the entities associated with the category; determining a background probability distribution for the occurrence of the terms in the set of queries; and comparing the probability distribution of the co-occurrence of the terms in the set of queries with the entities associated with the category and the background probability distribution to obtain the specificity value for the category; and classifying the category as a coherent category, wherein a coherent category is a category whose specificity value satisfies a threshold.
7. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: identifying a category from a knowledge base, the category including a plurality of entities; identifying a set of queries including terms related to the entities included in the category from a query stream; calculating a specificity value for the category, based at least on the entities included in the category and the set of queries, comprising: determining a probability distribution for co-occurrence of the terms in the set of queries with the entities associated with the category; determining a background probability distribution for the occurrence of the terms in the set of queries; and comparing the probability distribution of the co-occurrence of the terms in the set of queries with the entities associated with the category and the background probability distribution to obtain the specificity value for the category; and classifying the category as a coherent category, wherein a coherent category is a category whose specificity value satisfies a threshold. 10. The system of claim 7 , wherein the terms comprise individual words and aggregated sub-phrases.