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 equipment mounted on a shaft might use a male spline on the shaft that matches the female spline on the gear. The splines on the pictured drive shaft match with the feminine splines in the center of the clutch plate, as the smooth suggestion of the axle is definitely gear rack backed in the pilot bearing in the flywheel. An alternative to splines is usually a keyway and crucial, though splines give a longer fatigue life.[2]