The window sill is the bottom horizontal portion of the window. On a wood window this is a thick piece of wood, shaped to a slope of about 7 degrees, the purpose of which is to move water away from the bottom of the window sash where it could leak over a period of time. This can be commonly mistaken for a stool, which has also been provided within the glossary.
While most sills are sloped, they also can be flat. Sliding windows, for example, have flat sills and a section of track on the sill upon which the sash slides.