Splines are ridges or tooth[1][2][3] on a drive shaft that mesh with grooves in a mating piece and transfer torque to this, maintaining the angular correspondence between them.
For instance, a equipment mounted on a shaft might use a man spline on the shaft that matches the female spline on the apparatus. The splines on the pictured drive shaft match with the feminine splines in the heart of the gear rack clutch plate, while the 
smooth tip of the axle is usually backed in the pilot bearing in the flywheel. An alternative solution to splines is definitely a keyway and crucial, though splines provide a longer fatigue life.[2]