Extraordinary session:
Any meeting of the Conference of Parties or the Intergovernmental Committee taking place outside of ordinary sessions.  The Conference of Parties may meet in extraordinary session if it so decides or if the Intergovernmental Committee receives a request to that effect from at least one third of the Parties.  The Intergovernmental Committee can meet in extraordinary session at the request of at least two thirds of its Members.
Source: Rules of Procedure of the Conference of Parties.