The Collaborative International Dictionary of English v.0.48:

Lamprey \Lam"prey\ (l[a^]m"pr[y^]), n.; pl. Lampreys
   (l[a^]m"pr[i^]z). [OE. lampreie, F. lamproie, LL. lampreda,
   lampetra, from L. lambere to lick + petra rock, stone. The
   lampreys are so called because they attach themselves with
   their circular mouths to rocks and stones, whence they are
   also called rocksuckers. See Lap to drink, Petrify.]
   (Zool.)
   An eel-like marsipobranch of the genus Petromyzon, and
   allied genera; called also lamprey eel and lamper eel.
   The lampreys have a round, sucking mouth, without jaws, but
   set with numerous minute teeth, and one to three larger teeth
   on the palate (see Illust. of Cyclostomi). There are seven
   small branchial openings on each side. [Written also
   lamprel, and lampron.]
   [1913 Webster]

   Note: The common or sea lamprey of America and Europe
         (Petromyzon marinus), which in spring ascends rivers
         to spawn, is considered excellent food by many, and is
         sold as a market fish in some localities. The smaller
         river lampreys mostly belong to the genus
         Ammoc[oe]les, or Lampetra, as Ammoc[oe]les
         fluviatilis, of Europe, and Ammoc[oe]les
         [ae]pypterus of America. All lampreys attach
         themselves to other fishes, as parasites, by means of
         the suckerlike mouth.
         [1913 Webster]

WordNet (r) 3.0 (2006):

lamprey eel
    n 1: primitive eellike freshwater or anadromous cyclostome
         having round sucking mouth with a rasping tongue [syn:
         lamprey, lamprey eel, lamper eel]