Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

Scylla
     n : (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