18. The computing device of claim 17 , wherein extracting the plurality of the potential phrases comprises identifying ages of documents of the document corpus in which the plurality of the potential phrases occur.
17. A computing device comprising: a computer processor; and a tangible computer-readable storage medium storing instructions executed by the computer 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 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.