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