A room, usually only in a well-to-do Ancient Roman house, where one went to vomit between courses. In Ancient Rome it was fashionable to eat gluttonous amounts of food. The ultimate expression of this, usually by the upper crust, was to eat a meal so huge that one could not physically finish it unless one took breaks to vomit.
The vomitorium was the product of the accepted practice of bulemia.
Ancient Roman: I can't eat another bite. Excuse me while I visit the vomitorium. (Enter's vomitorium and pukes guts out) Ahhhh, much better. I'm ready for the second course!