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]

WordNet (r) 3.0 (2006):

Pediculus capitis
    n 1: infests the head and body of humans [syn: head louse,
         Pediculus capitis]