The Collaborative International Dictionary of English v.0.48:

Scylla \Scyl"la\, n.
   A dangerous rock on the Italian coast opposite the whirpool
   Charybdis on the coast of Sicily, -- both personified in
   classical literature as ravenous monsters. The passage
   between them was formerly considered perilous; hence, the
   saying "Between Scylla and Charybdis," signifying a great
   peril on either hand.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Scylla
    n 1: (Greek mythology) a sea nymph transformed into a sea
         monster who lived on one side of a narrow strait; drowned
         and devoured sailors who tried to escape Charybdis (a
         whirlpool) on the other side of the strait