noun
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.
猫が窓敷居の上で日向ぼっこをしている。
The cat is basking in the sun on the window sill.
窓敷居に小さな植木鉢を並べた。
I lined up small flowerpots on the window sill.