Spline \Spline\, n.

1. 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.

2. A long, flexble piece of wood sometimes used as a ruler.

Source: Webster's Revised Unabridged Dictionary (1913)