What is "batement"

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

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.

mid-15c., shortening of abatement.

n. abatement; diminution