rotunda
C2Substantiv/US /roʊtˈʌndə/ rōt-ŬND-ə; UK /roʊtˈʌndə/ rōt-ŬND-ə/
A round building; especially, one that is round both on the outside and inside, like the Pantheon at Less properly, but very commonly, used for a large round room; as, the rotunda of the Capitol at
