The Collaborative International Dictionary of English v.0.48:

Redif \Re*dif"\, n. [Turk. red[imac]f, fr. rad[imac]f, orig., he
   who rides behind another on the same beast, fr. radaf to
   follow.]
   A reserve force in the Turkish army, or a soldier of the
   reserve. See Army organization, above.
   [Webster 1913 Suppl.]