The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. A long, flexble piece of wood sometimes used as a ruler.
      [1913 Webster]

WordNet (r) 3.0 (2006):

spline
    n 1: a flexible strip (wood or rubber) used in drawing curved
         lines
    2: a thin strip (wood or metal) [syn: slat, spline]