11. A tangible computer-readable storage medium storing instructions that when executed by a processor cause the processor to perform steps comprising: extracting a plurality of potential phrases from a document corpus including a plurality of documents; assigning each of the plurality of potential phrases into a to a cluster from a plurality of clusters; identifying, for each of the plurality of clusters, a set of documents from the plurality of documents that include at least one potential phrase assigned to the cluster; determining, for each of the plurality of potential phrases, a score based on a number of documents that include the potential phrase from the set of documents identified for the cluster to which the potential phrase is assigned; selecting, from the plurality of potential phrases, potential phrases as dictionary phrases, each dictionary phrase selected based on the score determined for the dictionary phrase; extracting, for each dictionary phrase, a definition from the document corpus; and storing each dictionary phrase and the definition extracted for the selected dictionary phrase.
15. The computer-readable storage medium of claim 11 , wherein assigning each of the plurality of potential phrases to a cluster comprises determining co-occurrences of a pair of potential phrases from the plurality of potential phrases within the document corpus and determining similarity of concepts represented by the pair of potential phrases, the method further comprising: forming a similarity measure between the pair of potential phrases using the determined co-occurrences and the determined similarity of concepts; and applying machine learning to determine values by which to weight the determined co-occurrences and the determined similarity of concepts in order to produce the similarity measure.