The Collaborative International Dictionary of English v.0.48:

Leech \Leech\, n. [OE. leche, l[ae]che, physician, AS. l[=ae]ce;
   akin to Fries. l[=e]tza, OHG. l[=a]hh[imac], Icel.
   l[ae]knari, Sw. l[aum]kare, Dan. l[ae]ge, Goth. l[=e]keis,
   AS. l[=a]cnian to heal, Sw. l[aum]ka, Dan. l[ae]ge, Icel.
   l[ae]kna, Goth. l[=e]kin[=o]n.]
   1. A physician or surgeon; a professor of the art of healing.
      [Written also leach.] [Archaic] --Spenser.
      [1913 Webster]

            Leech, heal thyself.                  --Wyclif (Luke
                                                  iv. 23).

   2. (Zool.) Any one of numerous genera and species of annulose
      worms, belonging to the order Hirudinea, or Bdelloidea,
      esp. those species used in medicine, as Hirudo
      medicinalis of Europe, and allied species.
      [1913 Webster]

   Note: In the mouth of bloodsucking leeches are three
         convergent, serrated jaws, moved by strong muscles. By
         the motion of these jaws a stellate incision is made in
         the skin, through which the leech sucks blood till it
         is gorged, and then drops off. The stomach has large
         pouches on each side to hold the blood. The common
         large bloodsucking leech of America (Macrobdella
         decora) is dark olive above, and red below, with black
         spots. Many kinds of leeches are parasitic on fishes;
         others feed upon worms and mollusks, and have no jaws
         for drawing blood. See Bdelloidea. Hirudinea, and
         Clepsine.
         [1913 Webster]

   3. (Surg.) A glass tube of peculiar construction, adapted for
      drawing blood from a scarified part by means of a vacuum.
      [1913 Webster]

   Horse leech, a less powerful European leech (H[ae]mopis
      vorax), commonly attacking the membrane that lines the
      inside of the mouth and nostrils of animals that drink at
      pools where it lives.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bloodsucker \Blood"suck`er\, n.
   1. (Zool.) Any animal that sucks blood; esp., the leech
      (Hirudo medicinalis), and related species.
      [1913 Webster]

   2. One who sheds blood; a cruel, bloodthirsty man; one guilty
      of bloodshed; a murderer. [Obs.] --Shak.
      [1913 Webster]

   3. A hard and exacting master, landlord, or money lender; an
      extortioner.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Hirudo medicinalis
    n 1: large European freshwater leech formerly used for
         bloodletting [syn: medicinal leech, Hirudo medicinalis]