sill has definitions from the field of geology
[ noun ] structural member consisting of a continuous horizontal timber forming the lowest member of a framework or supporting structure

Used in print

(Sallie Bingham, "Moving Day," The Atlantic...)

He noticed a speck of dirt on the sill and swiped at it with his finger .

[ noun ] Last name, frequency rank in the U.S. is 8882
[ noun ] (geology) a flat (usually horizontal) mass of igneous rock between two layers of older sedimentary rock

Related terms

rock geology