1. A method performed by data processing apparatus, the method comprising: receiving a content item request requesting content items to be presented, at a user device, with search results for a first search query, the content item request including search query data specifying a geographic location that was referenced by the first search query and a search term of the first search query; determining, in response to receiving the content item request and by one or more data processing apparatus, that fewer than a threshold number of content items are eligible for presentation with search results responsive to the first search query; identifying, from a data store, in response to the determination that fewer than the threshold number of content items are eligible for presentation, and by one or more data processing apparatus, one or more terms that are each associated with at least the threshold number of content items that are eligible for presentation with search results responsive to search queries that include a reference to the geographic location, the one or more terms being different than the search term; selecting, for distribution in response to the received content item request and in response to the determination that fewer than the threshold number of content items are eligible for presentation with search results responsive to the first search query, one or more eligible content items using the one or more terms; and providing, in response to the received content item request, data that cause presentation, at the user device, of at least one of the one or more eligible content items selected using the one or more terms.
1. A method performed by data processing apparatus, the method comprising: receiving a content item request requesting content items to be presented, at a user device, with search results for a first search query, the content item request including search query data specifying a geographic location that was referenced by the first search query and a search term of the first search query; determining, in response to receiving the content item request and by one or more data processing apparatus, that fewer than a threshold number of content items are eligible for presentation with search results responsive to the first search query; identifying, from a data store, in response to the determination that fewer than the threshold number of content items are eligible for presentation, and by one or more data processing apparatus, one or more terms that are each associated with at least the threshold number of content items that are eligible for presentation with search results responsive to search queries that include a reference to the geographic location, the one or more terms being different than the search term; selecting, for distribution in response to the received content item request and in response to the determination that fewer than the threshold number of content items are eligible for presentation with search results responsive to the first search query, one or more eligible content items using the one or more terms; and providing, in response to the received content item request, data that cause presentation, at the user device, of at least one of the one or more eligible content items selected using the one or more terms. 9. The method of claim 1 , wherein: selecting one or more eligible content items comprises selecting eligible advertisements; and providing data that cause presentation of at least one of the one or more eligible content items comprises providing data that cause presentation of at least one of the eligible advertisements.