Rotunda \Ro*tun"da\, n. [Cf. It. rotonda, F. rotonde; both fr. L. rotundus round. See Rotund, a.]

(Arch.) A round building; especially, one that is round both on the outside and inside, like the Pantheon at Rome. Less properly, but very commonly, used for a large round room; as, the rotunda of the Capitol at Washington.

Source: Webster's Revised Unabridged Dictionary (1913)