Splines are ridges or teeth[1][2][3] on a drive shaft that mesh with grooves in a mating piece and transfer gear rack torque to it, maintaining the angular correspondence between them.
For instance, a equipment mounted on a shaft might use a male spline on the shaft that fits the female spline on the apparatus. The splines on the pictured drive shaft match with the female splines in the center of the clutch plate, while the smooth suggestion of the axle is supported in the pilot bearing in the flywheel. An alternative to splines is a keyway and crucial, though splines give a longer fatigue life.[2]