The Collaborative International Dictionary of English v.0.48:

Diadem \Di"a*dem\, n. [F. diad[`e]me, L. diadema, fr. Gr. ?, fr.
   ? to bind round; dia` through, across + ? to bind; cf. Skr.
   d[=a] to bind.]
   1. Originally, an ornamental head band or fillet, worn by
      Eastern monarchs as a badge of royalty; hence (later),
      also, a crown, in general. "The regal diadem." --Milton.
      [1913 Webster]

   2. Regal power; sovereignty; empire; -- considered as
      symbolized by the crown.
      [1913 Webster]

   3. (Her.) An arch rising from the rim of a crown (rarely also
      of a coronet), and uniting with others over its center.
      [1913 Webster]

   Diadem lemur. (Zool.) See Indri.

   Diadem spider (Zool.), the garden spider.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Diadem \Di"a*dem\, v. t.
   To adorn with a diadem; to crown.
   [1913 Webster]

         Not so, when diadem'd with rays divine.  --Pope.
   [1913 Webster]

         To terminate the evil,
         To diadem the right.                     --R. H. Neale.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diadem
    n 1: an ornamental jeweled headdress signifying sovereignty
         [syn: crown, diadem]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "diadem":
   O, anklet, annular muscle, annulus, areola, armlet, aureole,
   bangle, beads, bijou, bracelet, breastpin, brooch, cap of dignity,
   cap of maintenance, chain, chaplet, charm, chatelaine, circle,
   circuit, circumference, circus, closed circle, corona, coronet,
   crown, cycle, discus, disk, earring, ermine, eternal return,
   fairy ring, fob, garland, gem, glory, great seal, halo, jewel,
   lasso, locket, logical circle, loop, looplet, magic circle,
   necklace, noose, nose ring, orb, orbit, pin, precious stone,
   privy seal, purple, purple pall, radius, regalia, rhinestone, ring,
   robe of state, rod, rod of empire, rondelle, round, roundel,
   royal crown, saucer, scepter, seal, signet, sphincter, stickpin,
   stone, tiara, torque, triple plume, uraeus, vicious circle, wampum,
   wheel, wreath, wristband, wristlet



Easton's 1897 Bible Dictionary:

Diadem
   the tiara of a king (Ezek. 21:26; Isa. 28:5; 62:3); the turban
   (Job 29:14). In the New Testament a careful distinction is drawn
   between the diadem as a badge of royalty (Rev. 12:3; 13:1;
   19:12) and the crown as a mark of distinction in private life.
   It is not known what the ancient Jewish "diadem" was. It was the
   mark of Oriental sovereigns. (See CROWN.)