window sill; window ledge
Refers to the horizontal ledge at the bottom of a window, often used for placing small objects or as a structural part of the window frame.
The cat is basking in the sun on the window sill.
I lined up small flowerpots on the window sill.
Compound of 窓 (まど, 'window') and 敷居 (しきい, 'threshold' or 'sill'). The word 敷居 originally referred to the grooved threshold of a sliding door, and by extension came to mean any sill or ledge, including that of a window.