Bill Slawski started a thread over at Cre8asite Forums named Yahoo! Superunits: of signatures and co-occurence. In that thread, he discusses a new patent Yahoo! Search released (on April 14th) named Systems and methods for search processing using superunits.
Here is the patent's abstract:
In a search processing system, a concept network is generated from a set of queries by parsing the queries into units and defining various relationships between the units based in part on patterns of units that appear together in queries. Units in the concept network that have some similar characteristic(s) are grouped into superunits. For each superunit, there is a corresponding signature that defines the similar characteristic of the group. A query is processed by identifying constituent units, determining the superunit membership of some or all of the constituent units, and using that information to formulate a response to the query.
Bill tells us to look for "some new vocabulary words - a concept network, a unit, a superunit, and signatures." I briefly skimmed it, but the whole concept network seems very interesting. "A concept network is generated from a set of queries by parsing the queries into units and defining various relationships between the units, e.g., based on patterns of units that appear together in queries."