The Collaborative International Dictionary of English v.0.48:

Plateau \Pla*teau"\, n.; pl. F. Plateaux (F. ?; E. ?), E.
   Plateaus. [F., fr. OF. platel, properly a little plate. See
   Plate.]
   1. A flat surface; especially, a broad, level, elevated area
      of land; a table-land.
      [1913 Webster]

   2. An ornamental dish for the table; a tray or salver.
      [1913 Webster]