Source : Webster's Revised Unabridged Dictionary (1913)

Rein \Rein\, v. t. [imp. & p. p. {Reined} (r?nd); p. pr. & vb.
   n. {Reining}.]
   1. To govern or direct with the reins; as, to rein a horse
      one way or another.

            He mounts and reins his horse.        --Chapman.

   2. To restrain; to control; to check.

            Being once chafed, he can not Be reined again to
            temperance.                           --Shak.

   {To rein in} or {rein up}, to check the speed of, or cause to
      stop, by drawing the reins.

Rein \Rein\, v. i.
   To be guided by reins. [R.] --Shak.

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.

Source : WordNet®

rein
     n 1: one of a pair of long straps (usually connected to the bit
          or the headpiece) used to control a horse
     2: any means of control; "he took up the reins of government"

rein
     v 1: control and direct with or as if by reins; "rein a horse"
          [syn: {harness}, {rein in}, {draw rein}]
     2: stop or slow up one's horse or oneself by or as if by
        pulling the reins; "They reined in in front of the post
        office" [syn: {rein in}]
     3: stop or check by or as if by a pull at the reins; "He reined
        in his horses in front of the post office" [syn: {rein in}]
     4: keep in check; "rule one's temper" [syn: {rule}, {harness}]