Source : Webster's Revised Unabridged Dictionary (1913)

Preorder \Pre*or"der\, v. t.
   To order to arrange beforehand; to foreordain. --Sir W.
   Hamilton.

Source : Free On-Line Dictionary of Computing

pre-order
     
        1.  {traversal}.
     
        2.  A {relation} R is a pre-order if it is {reflexive}
        (x R x) and {transitive} (x R y R z => x R z).  If it is also
        {antisymmetric} (x R y R x => x = y) then it is a {partial
        ordering}.
     
        (2001-10-01)