Stool of a window , ∨ Window stool Meaning, Definition & Usage

Definitions
  1. (Arch.), the flat piece upon which the window shuts down, and which corresponds to the sill of a door; in the United States, the narrow shelf fitted on the inside against the actual sill upon which the sash descends. This is called a window seat when broad and low enough to be used as a seat.

Webster 1913