Conventions (Conferences) - participatory meeting designed for discussion, fact-finding, problem solving and consultation.  As compared with a congress, a conference is normally smaller in scale and more select in character - features which tend to facilitate the exchange of information. The term "conference" carries no special connotation as to frequency. Though not inherently limited in time, conferences are usually of limited duration with specific objectives.

SOURCE
IAPCO (the International Association of Professional Congress Organizers) "Meetings Industry Terminology".