A side event is a small tournament usually run in conjunction with a quizbowl tournament (the "main event"). Typically, a "side event" is partnered with an academic tournament so that it can draw in more players.
Side events usually cover a niche selection of subjects, while "main events" usually cover a standard distribution of all subjects (sometimes called "all-subject quizbowl"). Subject tournaments are common examples of side events. Colloquially, "side event" can also refer to a niche tournament or question set in itself (such as a vanity tournament), regardless of whether it is actually paired with a "main event."
Trash tournaments sometimes have certain attributes of side events (running in the evening or on Sunday after a larger academic tournament, providing 10 or fewer rounds, being unabashedly reflective of the authors' vanity interests) but are rarely categorized as "side events" unless they are further specialized to some subdistribution of trash. Similarly, official History Bowl events are not considered "side events."
The phenomenon of side events being announced without any real plan to actually write or hold them was satirized by Triacontakaipentagon in 2013.
Side event weekends
The difficulty of attaching side events to any given tournament has spawned side event weekends, in which primarily side events are played.
Side event weekends are held routinely in the Midwest, and the NorCal Weekend of Quizbowl has been held since 2016.