The Collaborative International Dictionary of English v.0.48:

Selion \Sel"ion\, n. [OF. seillon a measure of land, F. sillon a
   ridge, furrow, LL. selio a measure of land.]
   A short piece of land in arable ridges and furrows, of
   uncertain quantity; also, a ridge of land lying between two
   furrows. [Obs. or Prov. Eng.]
   [1913 Webster]