Trappist is not a style but a designation or appellation of origin, indicating that the beer in question was brewed at one of the seven recognized beer-producing Cistercian ("Trappist") monasteries in Belgium or the Netherlands. In every case, the beer is actually brewed on the monastery premises and under the supervision of the monks.
Most beer brewed by Trappists is in one of the standard Abbey beer styles, such as Dubbel or Tripel, or in a Specialty Belgian Beer style. However, technically, any beer brewed in an abbey in any style could be considered "Trappist" beer.
- Belgian Trappists and Abbey Beers - Brewing Techniques Magazine