19. The system of claim 17 , wherein the instructions cause the one or more computers to group initial labels that are associated with the set of matching web pages into a label group by performing operations comprising: identifying, among the set of initial labels, a first set of at least two initial labels that were obtained from a same domain; grouping the first set of at least two initial labels into a label group; identifying, among the set of initial labels, a second set of at least two initial labels that were obtained from a same domain; and grouping the second set of at least two initial labels into a label group.
17. A system comprising: a data store storing label data that specifies a set of initial labels for a non-text content item, wherein the non-text content item is associated with each of a plurality of web pages, and wherein each initial label includes one or more words; and one or more computers coupled to the data store, the one or more computers storing instructions that cause the one or more computers to interact with the data store and perform operations comprising: identifying a non-text content item that is associated with each of a plurality of web pages; receiving label data that includes a set of initial labels for the non-text content item, wherein each initial label includes one or more words; grouping, for each of two or more sets of matching web pages among the plurality of web pages, initial labels that are associated with the set of matching web pages into a label group, the initial labels for different set of matching web pages being grouped to different label groups; grouping different sets of matching labels from the set of initial labels into different label groups; and selecting, as a final label for the non-text content item, an n-gram of one or more words that is included in at least a threshold number of different label groups.