Leonardo da Vinci illustrated a ball bearing in mechanical studies that placed rolling balls between surfaces to reduce friction. His drawings show the basic principle that later became central to many rotating machines.
A ball bearing allows one part to rotate relative to another while small hardened balls roll between them. Rolling contact generally creates less resistance than sliding contact, making motion more efficient and reducing wear. Leonardo's studies explored ways to keep the balls separated so they would not bunch together.
Leonardo did not invent every bearing concept, and ancient and medieval engineers had already used rollers and other methods to reduce friction. His significance lies in the clarity of his mechanical drawings and his recognition of the practical problems involved in arranging rolling elements.
The modern ball bearing developed through later improvements in materials, manufacturing precision, cages, races, and lubrication. Leonardo's sketches were therefore an important early study, not a complete modern bearing ready for mass production.