Pl.: tubuli
Also: box-flue tile (esp. in Roman Britain)

Hollow ceramic tile, often box-like or cylindrical.In Roman-style baths, rows of superimposed tubuli were placed against the walls of heated rooms and connected to the hypocaust to create a heated wall. Chimney flues connected with the top row of tubuli ensured that the hot gases of the hypocaust were drawn into the tubuli. If the tubuli were not connected to a hypocaust, they served a mere insulation for the walls.

Tubuli in the Forum Baths of Ostia. They are embedded in a layer of (modern) mortar and would have been hidden behind a marble veneer, fragments of which are still in place (photo Konogan Beaufay).