What is a…
A mill such as mentioned in the Bible is a device used to grind grain into flour using stones. Small mills were powered by hand. Larger millstones were turned by people.
That used by the Hebrews consisted of two circular stones, each 2 feet in diameter and half a foot thick, the lower of which was called the “nether millstone” (Job 41:24) and the upper the “rider.” The upper stone was turned round by a stick fixed in it as a handle.
It was with the upper stone of a hand-mill that “a certain woman” at Thebez broke Abimelech's skull (Judges 9:53, “a piece of a millstone;” literally, “a millstone rider”, i.e., the “runner,” the stone which revolves. Compare 2 Samuel 11:21).