The Collaborative International Dictionary of English v.0.48:

Ranula \Ran"u*la\ (r[a^]n"[-u]*l[.a]), n. [L., a little frog, a
   little swelling on the tongue of cattle, dim. of rana a
   frog.] (Med.)
   A cyst formed under the tongue by obstruction of the duct of
   the submaxillary gland.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ranula
    n 1: a cyst on the underside of the tongue