What does "sill" mean or refer to?

What is or what is the meaning of the “Sill” in the Lawton - Ft. Sill area of OK?

I heard reference made to it recently and haven’t been able to track it down since.

From “A History of Fort Sill:”

*At first the garrison was called “Camp Wichita” and referred to by the Indians as “the Soldier House at Medicine Bluffs.” Sheridan later named it in honor of his West Point classmate and friend, Brigadier General Joshua W. Sill, who was killed during the Civil War. *