18. A system, comprising: a data store storing a topic dictionary comprising topic words related to a topic; a topic word processing module configured to: determine a topic word divergence value based on a topic word, a document corpus and a topic document corpus, wherein the topic document corpus is a corpus of topic documents related to a topic, the document corpus is a corpus of documents that includes the topic documents and other documents, and the topic word is a word in a topic dictionary that is related to the topic; select a candidate topic word that is not a word in the topic dictionary; determine a candidate topic word divergence value for the candidate topic word based on the document corpus and the topic document corpus; and determine whether the candidate topic word is a topic word for the topic based on the candidate topic word divergence value and the topic word divergence value; and a dictionary updater module configured to store the candidate topic word in the topic dictionary if the candidate topic word is determined to be a topic word.
18. A system, comprising: a data store storing a topic dictionary comprising topic words related to a topic; a topic word processing module configured to: determine a topic word divergence value based on a topic word, a document corpus and a topic document corpus, wherein the topic document corpus is a corpus of topic documents related to a topic, the document corpus is a corpus of documents that includes the topic documents and other documents, and the topic word is a word in a topic dictionary that is related to the topic; select a candidate topic word that is not a word in the topic dictionary; determine a candidate topic word divergence value for the candidate topic word based on the document corpus and the topic document corpus; and determine whether the candidate topic word is a topic word for the topic based on the candidate topic word divergence value and the topic word divergence value; and a dictionary updater module configured to store the candidate topic word in the topic dictionary if the candidate topic word is determined to be a topic word. 19. The system of claim 18 , wherein the topic word processing module is configured to: determine a first probability associated with the candidate topic word in the topic document corpus; determine a second probability associated with the candidate topic word in the document corpus; and calculate the candidate topic word divergence value based on a ratio of the first probability to the product of the second probability and a logarithmic value based on the first probability.