elasticsearch/elasticsearch — pull request #5146 by markharwood (2014-02-17 15:58:52+00:00)
Title: Significant_terms aggregation 

A new aggregation that identifies terms that are significant rather than merely popular in a result set.

Significance is related to the changes in document frequency observed between everyday use in the corpus and frequency observed in the result set. The asciidocs include extensive details on the various applications of this feature.