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.
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. 14. The computer-readable storage medium of claim 11 , wherein the instructions further cause the processor to perform steps comprising: receiving a request to define a phrase associated with an electronic book; identifying the requested phrase within the dictionary phrases stored; and providing the stored definition associated with the requested phrase.