7. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: selecting a pair of words; receiving a document having an associated title or anchor; determining that a word of the pair of words occurs in the title or anchor of the document and that a different word of the pair of words occurs within the document; determining a co-occurrence frequency of the word of the pair of words and the different word of the pair of words in a collection of documents; and determining that the word and the different word are synonyms based at least upon determining that the word of the pair of words occurs in the title or anchor of the document and that the different word of the pair of words occurs within the document and based at least on the determined co-occurrence frequency of the word of the pair of words and the different word of the pair of words.
7. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: selecting a pair of words; receiving a document having an associated title or anchor; determining that a word of the pair of words occurs in the title or anchor of the document and that a different word of the pair of words occurs within the document; determining a co-occurrence frequency of the word of the pair of words and the different word of the pair of words in a collection of documents; and determining that the word and the different word are synonyms based at least upon determining that the word of the pair of words occurs in the title or anchor of the document and that the different word of the pair of words occurs within the document and based at least on the determined co-occurrence frequency of the word of the pair of words and the different word of the pair of words. 8. The system of claim 7 , wherein the anchor is a Uniform Resource Locator (URL) anchor.