Pinion

A pinion is a round gear—usually the smaller of two meshed gears—used in several applications, including drivetrain and rack and pinion systems.
Applications.
Drivetrain.
Drivetrains usually feature a gear known as the pinion, which may vary in different systems, including
Rack and pinion.
In rack and pinion system, the pinion is the round gear that engages and moves along the linear rack.