A large public meeting event for the purposes of enjoying a similar interest as a larger group.
Conventions are typically held in large meeting areas where like-minded people have a chance to converse about similar interests. Conventions may also have items at them that are normally unavailable to the greater public.
RPG sessions are sometimes played at conventions. Convention games differ from regular games in that they are often a single-session, played by people who did not previously know each other, and often require both players and DMs have a stronger commitment to playing things as-written.