The mechanical element Leonardo da Vinci sketched in a form often regarded as an early ball-bearing design was a ball bearing.
In his Codex Atlanticus, Leonardo drew systems using balls to reduce friction between moving surfaces. His arrangement separated the balls so they would not rub together, an important insight for making a rotating mechanism work more smoothly. The design is frequently cited as an early ancestor of the modern ball bearing.
A ball bearing uses rolling contact instead of the sliding contact found in a plain bearing. That substitution can reduce friction and wear, although the result depends on materials, lubrication, alignment, and load. Leonardo’s sketch was a mechanical concept, not a mass-produced standardized bearing like those used in modern machines.
The invention is sometimes described as Leonardo inventing the ball bearing outright. A more careful statement is that he recorded an early practical bearing design. Earlier rolling devices existed, and modern bearings required later manufacturing techniques and precision engineering.