Ad Hoc Select Committee.  A temporary committee selected for a special purpose or to deal with a specific subject. Conference committees are ad hoc joint committees. A House rule adopted in 1975 authorizes the Speaker to refer measures to special ad hoc committees, appointed by him, with the approval of the House. These committees are supposed to consider measures that overlap the jurisdictions of several standing committees.  

— From “Congressional Quarterly’s American Congressional Dictionary” by Walter Kravitz.