The circuit is the collective name for the group of schools and teams that regularly participate in weekend quizbowl tournaments.
Circuit vs. Non-Circuit
By general consensus, "the circuit" does not include schools that only participate in local quizbowl leagues or local TV tournaments even if such events are held on pyramidal questions. Such schools are, however, excellent candidates to join the circuit simply by participating in weekend quizbowl tournaments in addition to those leagues and TV tournaments. A key part of outreach is finding ways to convince more of these non-circuit but already extant teams to join the circuit.
Due to geographic restrictions, "the circuit" is divided into several more-or-less autonomous regions, each of which is connected by common national tournaments or larger regional tournaments that teams travel to outside their primary area. When referring to the circuit or one of its divisions, it is proper to call it by its name of "the circuit" or "the [insert geographic division here] circuit", rather than just saying "circuit".
The High School Circuit
High school quizbowl is largely divided into a separate circuit for each state, although there is substantial overlap between circuits on the east coast (thus the creation of a historically-strong DC-area circuit that may overlap with the Eastern Pennsylvania or New York/New Jersey circuits depending on the specific event). California is largely divided into the Northern California and Southern California circuits and Pennsylvania between the Western and Eastern Pennsylvania circuits (see also geographic exclusivity). Some circuits (such as Texas and Missouri) have a nominal head, such as the president of the local quizbowl alliance or coaches' association, whereas others have no unifying association and set their schedules more haphazardly. Several prestigious tournaments, such as WUHSAC, Harvard Fall Tournament, and the Texas Invitational draw many teams from outside the immediate circuit of the region who seek to play a more competitive or diverse field.
The College Circuit
In college, the increased geographic spacing between teams usually means that circuits form in multi-state regions rather than individual state associations. Each circuit usually holds its own mirrors of the tournaments produced each year, though some tournaments (such as older Penn Bowls or VCU Closed) are hosted with an eye to cross-circuit participation.
Regional college circuits have historically existed in the Northeast (Boston area + New England + New York), the Mid-Atlantic, the Southeast, the upper Midwest, Missouri/the lower Midwest, Texas, California, the Pacific Northwest, and Canada (Ontario/Quebec). These circuits are not set in stone -- teams caught in between or on the border of two circuits (such as Penn and Princeton, between the Northeast and Mid-Atlantic) often alternate between attending sites in one of the nearby circuits or the other, and sometimes teams travel a longer distance from their local area for greater competition.
As of 2013, the most active college circuits are in the Northeast (Boston area to New York), the mid-Atlantic, and the Midwest; for this reason, at least one collegiate national championship is held in those circuits each year to minimize travel distance for a large number of strong teams.
Middle School Circuit
The middle school quiz bowl circuit largely resembles the high school circuit.