The Collaborative International Dictionary of English v.0.48:

Louse \Louse\ (lous), n.; pl. Lice (l[imac]s). [OE. lous, AS.
   l[=u]s, pl. l[=y]s; akin to D. luis, G. laus, OHG. l[=u]s,
   Icel. l[=u]s, Sw. lus, Dan. luus; perh. so named because it
   is destructive, and akin to E. lose, loose.] (Zool.)
   1. Any one of numerous species of small, wingless, suctorial,
      parasitic insects belonging to a tribe (Pediculina), now
      usually regarded as degraded Hemiptera. To this group
      belong of the lice of man and other mammals; as, the head
      louse of man (Pediculus capitis), the body louse
      (Pediculus vestimenti), and the crab louse (Phthirius
      pubis), and many others. See Crab louse, Dog louse,
      Cattle louse, etc., under Crab, Dog, etc.
      [1913 Webster]

   2. Any one of numerous small mandibulate insects, mostly
      parasitic on birds, and feeding on the feathers. They are
      known as Mallophaga, or bird lice, though some occur on
      the hair of mammals. They are usually regarded as degraded
      Pseudoneuroptera. See Mallophaga.
      [1913 Webster]

   3. Any one of the numerous species of aphids, or plant lice.
      See Aphid.
      [1913 Webster]

   4. Any small crustacean parasitic on fishes. See
      Branchiura, and Ichthvophthira.
      [1913 Webster]

   Note: The term is also applied to various other parasites;
         as, the whale louse, beelouse, horse louse.
         [1913 Webster]

   Louse fly (Zool.), a parasitic dipterous insect of the
      group Pupipara. Some of them are wingless, as the bee
      louse.

   Louse mite (Zool.), any one of numerous species of mites
      which infest mammals and birds, clinging to the hair and
      feathers like lice. They belong to Myobia,
      Dermaleichus, Mycoptes, and several other genera.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Crab \Crab\ (kr[a^]b), n. [AS. crabba; akin to D. krab, G.
   krabbe, krebs, Icel. krabbi, Sw. krabba, Dan. krabbe, and
   perh. to E. cramp. Cf. Crawfish.]
   1. (Zool.) One of the brachyuran Crustacea. They are mostly
      marine, and usually have a broad, short body, covered with
      a strong shell or carapace. The abdomen is small and
      curled up beneath the body.
      [1913 Webster]

   Note: The name is applied to all the Brachyura, and to
         certain Anomura, as the hermit crabs. Formerly, it was
         sometimes applied to Crustacea in general. Many species
         are edible, the blue crab of the Atlantic coast being
         one of the most esteemed. The large European edible
         crab is Cancer padurus. Soft-shelled crabs are blue
         crabs that have recently cast their shells. See
         Cancer; also, Box crab, Fiddler crab, Hermit
         crab, Spider crab, etc., under Box, Fiddler.
         etc.
         [1913 Webster]

   2. The zodiacal constellation Cancer.
      [1913 Webster]

   3. [See Crab, a.] (Bot.) A crab apple; -- so named from its
      harsh taste.
      [1913 Webster]

            When roasted crabs hiss in the bowl,
            Then nightly sings the staring owl.   --Shak.
      [1913 Webster]

   4. A cudgel made of the wood of the crab tree; a crabstick.
      [Obs.] --Garrick.
      [1913 Webster]

   5. (Mech.)
      (a) A movable winch or windlass with powerful gearing,
          used with derricks, etc.
      (b) A form of windlass, or geared capstan, for hauling
          ships into dock, etc.
      (c) A machine used in ropewalks to stretch the yarn.
      (d) A claw for anchoring a portable machine.
          [1913 Webster]

   Calling crab. (Zool.) See Fiddler., n., 2.

   Crab apple, a small, sour apple, of several kinds; also,
      the tree which bears it; as, the European crab apple
      (Pyrus Malus var. sylvestris); the Siberian crab apple
      (Pyrus baccata); and the American (Pyrus coronaria).


   Crab grass. (Bot.)
      (a) A grass (Digitaria sanguinalis syn. Panicum
          sanguinalis); -- called also finger grass.
      (b) A grass of the genus Eleusine (Eleusine Indica);
          -- called also dog's-tail grass, wire grass, etc.


   Crab louse (Zool.), a species of louse (Phthirius pubis),
      sometimes infesting the human body.

   Crab plover (Zool.), an Asiatic plover (Dromas ardeola).


   Crab's eyes, or Crab's stones, masses of calcareous
      matter found, at certain seasons of the year, on either
      side of the stomach of the European crawfishes, and
      formerly used in medicine for absorbent and antacid
      purposes; the gastroliths.

   Crab spider (Zool.), one of a group of spiders
      (Laterigrad[ae]); -- called because they can run
      backwards or sideways like a crab.

   Crab tree, the tree that bears crab applies.

   Crab wood, a light cabinet wood obtained in Guiana, which
      takes a high polish. --McElrath.

   To catch a crab (Naut.), a phrase used of a rower:
      (a) when he fails to raise his oar clear of the water;
      (b) when he misses the water altogether in making a
          stroke.
          [1913 Webster]

WordNet (r) 3.0 (2006):

Phthirius pubis
    n 1: a louse that infests the pubic region of the human body
         [syn: crab louse, pubic louse, crab, Phthirius
         pubis]