French Casement

A casement styled unit with two sash in one frame providing a sense of openness unrestricted by a vertical mullion or stiles when both sash are open.

Casement_window (Wikipedia)
Casement window, with latticed lights

A casement is a window that is attached to its frame by one or more hinges at the side. They are used singly or in pairs within a common frame, in which case they are hinged on the outside. Casement windows are often held open using a casement stay. Windows hinged at the top are referred to as awning windows, and ones hinged at the bottom are called hoppers.