The windowsill is the flat ledge along the bottom of a window, like a narrow shelf. Most windows have one inside and one outside. The inside one is where people put small plants, a cup of tea, a candle, a cat. The outside one slopes slightly downward on purpose, so rain runs off instead of soaking into the wall — that is what it was built for, though nobody thinks about it. Because it is such an ordinary domestic thing, the word shows up constantly in stories and in daily talk: sitting on the windowsill, leaving keys on the windowsill, a plant dying on the windowsill because nobody watered it. You will also see it written as two words, 'window sill', and as 'window-sill'. All three are fine; one word is the most common now. In building work people also say just 'sill' — and a 'sill' can be the bottom part of a door frame too, so the 'window' half is doing real work in this word. In Taiwan, many flats have deep concrete windowsills wide enough to sit on, which is exactly the picture this word wants.