draft bead, deep bead, sill bead, ventilating bead, window bead
A small fillet or strip which is fixed to the sill of a double-hung window; permits ventilation at the meeting rail while avoiding a draft at the sill; also called a draft stop.
In a double-hung window, a strip of wood fixed to the casing, along the inner edge of the inner sash; restricts the motion of the sash to a vertical plane.