Source : Webster's Revised Unabridged Dictionary (1913)

Lierne rib \Lierne" rib`\ (ly[^a]rn" r[i^]b`). [F. lierne.]
   (Arch.)
   In Gothic vaulting, any rib which does not spring from the
   impost and is not a ridge rib, but passes from one boss or
   intersection of the principal ribs to another.