The correct term is spine and it is the stiffness of the arrow.There is static spine,(how much a shaft flexes when checked on a spine tester) and then there is dynamic spine which is what the spine does when shot.


An arrow has to be stiff enough to handle the stress of being shot while still having some flex and vibration.You get too light on spine and the shaft will flex uncontrollably when shot and is impossible to shoot accurately.


Stiff arrow does just the opposite,doesn't absorb all the energy from the bow and has little to no flex or vibration when shot.Harder to tune but can be shot extremely accurately with a small amount of forgiveness lost.