window box

a box for growing plants, placed at or in a window.

a hollow space in a window frame for a sash weight.

Origin of window box

First recorded in 1880–85

Then I planted them in a corner of the window-box that I had left for them.

Cornell Nature-Study Leaflets |Various
And to our great joy the window-box was in position when we got home again.

Punch, or the London Charivari, May 27, 1914 |Various
From a window-box she took a pinch of dry soil and dropped it into the bosom of her gown.

The Slayer Of souls |Robert Chambers
The length of a window-box will depend on the size of the window.

What Shall We Do Now?: Five Hundred Games and Pastimes |Dorothy Canfield Fisher

The vulgar speak of it as a window-box; that is how one knows that they are vulgar.

If I May |A. A. Milne

window box

a long narrow box, placed on or outside a windowsill, in which plants are grown

either of a pair of vertical boxes, attached to the sides of a sash window frame, that enclose a sash cord and counterbalancing weight