The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rotunda
    n 1: a building having a circular plan and a dome
    2: a large circular room