IAPCO (the International Association of Professional Congress Organizers) publishes a book called "Meetings Industry Terminology" which functions as a dictionary for the meetings industry. These are the definitions as put out by IAPCO:
Meeting - general term indicating the coming together of a number of people in one place, to confer or carry out a particular activity. Frequency: can be on an ad hoc basis or according to a set pattern, as for instance annual general meetings, commitee meetings, etc.
Incentive - meeting event as part of a programme which is offered to its participants to reward a previous performance.
Conference - 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.
Exhibition - Events at which products and services are displayed.
Recently, there has been an industry driven initiative to not use the "MICE Market" label and instead say "The Meetings Industry" which encompasses all the above.