Splines are ridges or the 
teeth[1][2][3] on a drive shaft that mesh with grooves in a planetary gearbox mating piece and transfer torque to this, maintaining the angular correspondence between them.
For instance, a gear mounted on a shaft might use a man spline on the shaft that fits the female spline on the gear. The splines on the pictured drive shaft match with the female splines in the center of the clutch plate, as the smooth tip of the axle is supported in the pilot bearing in the flywheel. An alternative to splines is usually a keyway and essential, though splines provide a longer fatigue life.[2]