Rotunda
When a room or building has cylindrical walls, it's a rotunda
Rotundas are round rooms or buildings, sometimes with a domed roof. The word "rotunda" has its roots in the Latin word "rotonda," meaning "round"; these structures became popular in medieval Central Europe. Curves in architecture always require precisely angled cuts and extra materials and engineering, so these major feats of architecture were first used in churches, libraries, government buildings, museums and halls as showpieces.
Rotundas have cylindrical walls and most commonly a domed roof. Dormers are punched into the domed ceiling of this rotunda to let light in.
Cove lights circle the dome of this rotunda, and pin lights create a starry-sky effect.
A semicircular domed or vaulted room off a main structure of a building is called an apse. Apses are commonly found in churches; some date back to the 14th century.
Even though the ceiling is not domed, this room can still be considered a rotunda because the walls are cylindrical.
This rotunda has a metal domed roof that's similar to an onion roof.
These dome houses would not be called rotundas, because the walls are not cylindrical; they're spherical.