13. A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that when executed by data processing apparatus cause the data processing apparatus to perform operations comprising: receiving label data that specifies a set of initial labels for a non-text content item and a resource identifier for each initial label, wherein each initial label includes one or more words; selecting one or more sets of matching web pages from a plurality of web pages, wherein each set of matching web pages includes two or more matching web pages; grouping, for each set of matching web pages, initial labels that are associated with the set of matching web pages into a separate initial label group that corresponds to the set of matching web pages; selecting one or more sets of matching labels, wherein each set of matching labels includes two or more initial labels; grouping each set of matching labels into a separate initial label group that corresponds to the set of matching labels; and selecting, as a final label for the non-text content item, an n-gram of one or more words that are included in at least a threshold number of separate initial label groups.
13. A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that when executed by data processing apparatus cause the data processing apparatus to perform operations comprising: receiving label data that specifies a set of initial labels for a non-text content item and a resource identifier for each initial label, wherein each initial label includes one or more words; selecting one or more sets of matching web pages from a plurality of web pages, wherein each set of matching web pages includes two or more matching web pages; grouping, for each set of matching web pages, initial labels that are associated with the set of matching web pages into a separate initial label group that corresponds to the set of matching web pages; selecting one or more sets of matching labels, wherein each set of matching labels includes two or more initial labels; grouping each set of matching labels into a separate initial label group that corresponds to the set of matching labels; and selecting, as a final label for the non-text content item, an n-gram of one or more words that are included in at least a threshold number of separate initial label groups. 14. The non-transitory computer storage medium of claim 13 , wherein receiving label data specifying a set of initial labels for a non-text content item comprises receiving a set of initial labels for content that is provided in a non-text format.