A Perfecta is US term for a type of bet in which the bettor must predict the first and second place winners in a given competition (such as a horse race, though any contest with multiple contenders is fair game). This is similar to both the trifecta (in which one attempts to predict the first three winners in correct order) and the Quiniela (in which one need only predict the first two winners in any order).
In Spanish, the Perfecta is known as Quiniela Perfecta. Perfecta is also sometimes called the exacta.