window sill; window ledge
Refers to the horizontal ledge at the bottom of a window, often used for placing small objects or plants.
The cat is basking in the sun on the window sill.
窓台に小さな植木鉢を置いた。
I placed a small flowerpot on the window sill.
A compound of 窓, “window,” and 台, “stand, base, or ledge.” 台 has its regular Sino-Japanese reading だい here.