batement light

from 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]