20. The system of claim 14 , wherein the measure of importance of a term is computed by: determining terms that co-occur with the term in received search queries; computing an inverse document frequency of each term that co-occurs with the term in received search queries; and computing the measure of importance based at least in part on a sum of the computed inverse document frequency of each term that co-occurs with the term in received search queries.
14. The system of claim 13 , wherein computing a score for the second term comprises: computing respective changes in co-occurrence frequency between corresponding elements of the first vector and the second vector; generating an order of co-occurring terms according to the corresponding computed changes in co-occurrence frequency; and computing a measure of importance of a top number of co-occurring terms in the order.