Source : Webster's Revised Unabridged Dictionary (1913)

Sea slug \Sea" slug`\ (Zo["o]l.)
   (a) A holothurian.
   (b) A nudibranch mollusk.



   7. (Print.) A thick strip of metal less than type high, and
      as long as the width of a column or a page, -- used in
      spacing out pages and to separate display lines, etc.

   {Sea slug}. (Zo["o]l.)
      (a) Any nudibranch mollusk.
      (b) A holothurian.

   {Slug caterpillar}. Same as {Slugworm}.

Source : WordNet®

sea slug
     n : any of various marine gastropods of the suborder
         Nudibranchia having a shell-less and often beautifully
         colored body [syn: {nudibranch}]