The stibadium is a later form of the Roman Triclinium. Different because of its slightly more semi-circular shape. Often in movies about Ancient Roman Convivia, they actually feature a stibadium rather than a triclinium.
The stibadium was originally an outdoor couch but was introduced indoors in the 2nd-3rd centuries B.C. as the shape made it easier for entertainment.
Rogue Classicism [1]