Abstract: Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for collecting query term substitution data based on one or more identified concepts. According to one implementation, a method includes receiving a query that includes at least three sequential query terms; determining that the sequential query terms represent a concept; and in response to determining that the sequential query terms represent a concept, collecting query term substitution data for one or more query terms that occur in queries that include the concept.

Abstract: Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for retrieving documents. One of the methods includes receiving a search query that includes a first query term and an adjacent, second query term, and a substitute term for the first query term. A determination is made that the first query term and the substitute term satisfy one or more predetermined criteria and that a resource does not include the first query term. The resource is selected to be scored only if the substitute term occurs adjacent to the second term in the resource.

Abstract: Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for receiving a search query that includes a query term from a client device; obtaining first search results for the search query; identifying a candidate expansion of the query term in text associated with the first search results; revising the search query to include the candidate expansion of the query term; and obtaining second search results for the revised search query.

Abstract: Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for identifying and scoring restricted-locality synonyms. In one aspect, a method includes receiving a search query including a query term and a synonym for the query term. The synonym is evaluated using one or more predetermined criteria and based on the evaluation is designated as a restricted-locality synonym. A first scoring model is selected that specifies how to score occurrences of restricted-locality synonyms in documents. A document is received that is identified as being responsive to the search query. A document score for the document is determined using the first scoring model.

Abstract: Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for identifying and scoring restricted-locality synonyms. In one aspect, a method includes receiving a search query including a query term and a synonym for the query term. The synonym is evaluated using one or more predetermined criteria and based on the evaluation is designated as a restricted-locality synonym. A first scoring model is selected that specifies how to score occurrences of restricted-locality synonyms in documents. A document is received that is identified as being responsive to the search query. A document score for the document is determined using the first scoring model.