window box

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A usually long, narrow box for growing plants, placed on or hung from a windowsill or ledge.
• noun One of the vertical grooves on the inner sides of a window frame for the weights that counterbalance the sash.

from The Century Dictionary.

• noun In carpentry, the vertical hollow upright of the window-frame prepared to hold the weights for a sliding sash.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun a narrow box, filled with earth, placed on a windowsill and used for growing plants

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a long narrow box for growing plants on a windowsill