A rectangular piece fitting grooves like key seats in a hub and a shaft, so that while the one may slide endwise on the other, both must revolve together; a feather; also, sometimes, a groove to receive such a rectangular piece.
A long, flexble piece of wood sometimes used as a ruler.
Webster's Revised Unabridged Dictionary (1913), edited by Noah Porter. About
In order to secure one object on a shaft, longitudinal grooves or splines are cut all around the shaft with matching grooves in the object. In this way the object slides on the shaft for installation so that both the object and the shaft rotate together.