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.
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   2. Regal power; sovereignty; empire; -- considered as
      symbolized by the crown.
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   3. (Her.) An arch rising from the rim of a crown (rarely also
      of a coronet), and uniting with others over its center.
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   Diadem lemur. (Zool.) See Indri.

   Diadem spider (Zool.), the garden spider.
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The Collaborative International Dictionary of English v.0.48:

Diadem \Di"a*dem\, v. t.
   To adorn with a diadem; to crown.
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         Not so, when diadem'd with rays divine.  --Pope.
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         To terminate the evil,
         To diadem the right.                     --R. H. Neale.
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WordNet (r) 3.0 (2006):

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