Source : Webster's Revised Unabridged Dictionary (1913)

Rein \Rein\ (r?n), n. [F. r[^e]ne, fr. (assumed) LL. retina, fr.
   L. retinere to hold back. See {Retain}.]
   1. The strap of a bridle, fastened to the curb or snaffle on
      each side, by which the rider or driver governs the horse.

            This knight laid hold upon his reyne. --Chaucer.

   2. Hence, an instrument or means of curbing, restraining, or
      governing; government; restraint. ``Let their eyes rove
      without rein.'' --Milton.

   {To give rein}, {To give the rein to}, to give license to; to
      leave withouut restrain.

   {To take the reins}, to take the guidance or government; to
      assume control.