The Collaborative International Dictionary of English v.0.48:

Bedbug \Bed"bug`\, n. (Zool.)
   A wingless, bloodsucking, hemipterous insect (Cimex
   Lectularius), sometimes infesting houses and especially
   beds. See Illustration in Appendix.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Bug \Bug\ (b[u^]g), n. [OE. bugge, fr. W. bwg, bwgan, hobgoblin,
   scarecrow, bugbear. Cf. Bogey, Boggle.]
   1. A bugbear; anything which terrifies. [Obs.]
      [1913 Webster]

            Sir, spare your threats:
            The bug which you would fright me with I seek.
                                                  --Shak.
      [1913 Webster]

   2. (Zool.) A general name applied to various insects
      belonging to the Hemiptera; as, the squash bug; the chinch
      bug, etc.
      [1913 Webster]

   3. (Zool.) An insect of the genus Cimex, especially the
      bedbug (Cimex lectularius). See Bedbug.
      [1913 Webster]

   4. (Zool.) One of various species of Coleoptera; as, the
      ladybug; potato bug, etc.; loosely, any beetle.
      [1913 Webster]

   5. (Zool.) One of certain kinds of Crustacea; as, the sow
      bug; pill bug; bait bug; salve bug, etc.
      [1913 Webster]

   Note: According to popular usage in England and among
         housekeepers in America around 1900, bug, when not
         joined with some qualifying word, was used specifically
         for bedbug. As a general term it is now used very
         loosely in America as a colloquial term to mean any
         small crawling thing, such as an insect or arachnid,
         and was formerly used still more loosely in England.
         "God's rare workmanship in the ant, the poorest bug
         that creeps." --Rogers (--Naaman). "This bug with
         gilded wings." --Pope.
         [1913 Webster +PJC]

   6. (Computers) An error in the coding of a computer program,
      especially one causing the program to malfunction or fail.
      See, for example, year 2000 bug. "That's not a bug, it's
      a feature!"
      [PJC]

   7. Any unexpected defect or flaw, such as in a machine or a
      plan.
      [PJC]

   8. A hidden electronic listening device, used to hear or
      record conversations surreptitiously.
      [PJC]

   9. An infectious microorganism; a germ[4]. [Colloq.]
      [PJC]

   10. An undiagnosed illness, usually mild, believed to be
       caused by an infectious organism. [Colloq.]

   Note: In some communities in the 1990's, the incidence of
         AIDS is high and AIDS is referred to colloquially as
         "the bug".
         [PJC]

   11. An enthusiast; -- used mostly in combination, as a camera
       bug. [Colloq.]
       [PJC]

   Bait bug. See under Bait.

   Bug word, swaggering or threatening language. [Obs.]
      --Beau. & Fl.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bedbug
    n 1: bug of temperate regions that infests especially beds and
         feeds on human blood [syn: bedbug, bed bug, chinch,
         Cimex lectularius]