The Collaborative International Dictionary of English v.0.48:

Batement \Bate"ment\, n. [For Abatement. See 2d Bate.]
   Abatement; diminution. --Moxon.
   [1913 Webster]

   Batement light (Arch.), a window or one division of a
      window having vertical sides, but with the sill not
      horizontal, as where it follows the rake of a staircase.
      [1913 Webster]