Splines are ridges or 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 man spline on the shaft that fits the feminine 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 planetary gearbox suggestion of the axle is definitely supported in the pilot bearing in the flywheel. An alternative solution to splines can be a keyway and crucial, though splines give a longer fatigue life.[2]