The Collaborative International Dictionary of English v.0.48:

Baldachin \Bal"da*chin\, n. [LL. baldachinus, baldechinus, a
   canopy of rich silk carried over the host; fr. Bagdad, It.
   Baldacco, a city in Turkish Asia from whence these rich silks
   came: cf. It. baldacchino. Cf. Baudekin.]
   1. A rich brocade; baudekin. [Obs.]
      [1913 Webster]

   2. (Arch.) A structure in form of a canopy, sometimes
      supported by columns, and sometimes suspended from the
      roof or projecting from the wall; generally placed over an
      altar; as, the baldachin in St. Peter's.
      [1913 Webster]

   3. A portable canopy borne over shrines, etc., in procession.
      [1913 Webster] [Written also baldachino, baldaquin,
      etc.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

baldachin
    n 1: ornamented canopy supported by columns or suspended from a
         roof or projected from a wall (as over an altar)