The Collaborative International Dictionary of English v.0.48:

Rein \Rein\, v. i.
   To be guided by reins. [R.] --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            This knight laid hold upon his reyne. --Chaucer.
      [1913 Webster]

   2. Hence, an instrument or means of curbing, restraining, or
      governing; government; restraint. "Let their eyes rove
      without rein." --Milton.
      [1913 Webster]

   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.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            He mounts and reins his horse.        --Chapman.
      [1913 Webster]

   2. To restrain; to control; to check.
      [1913 Webster]

            Being once chafed, he can not
            Be reined again to temperance.        --Shak.
      [1913 Webster]

   To rein in or To rein up,
      (a) to check the speed of, or cause to stop, by drawing
          the reins. Hence,
      (a) to cause (a person) to slow down or cease some
          activity; -- to rein in is used commonly of superiors
          in a chain of command, ordering a subordinate to
          moderate or cease some activity deemed excessive.
          [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

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"
    v 1: control and direct with or as if by reins; "rein a horse"
         [syn: harness, rein in, draw rein, 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, 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,
       rein in]
    4: keep in check; "rule one's temper" [syn: rule, harness,
       rein]

Moby Thesaurus II by Grady Ward, 1.0:

115 Moby Thesaurus words for "rein":
   administration, arrest, arrestation, backpedal, backwater, brake,
   bridle, check, clip the wings, collect, command, constrain,
   constraint, contain, control, cool, cool off, cooling,
   cooling down, cooling off, curb, curtail, curtailment, decelerate,
   deceleration, delay, detain, dompt, draw rein, ease off, ease up,
   enjoin, govern, guard, harness, helm, hinder, hindrance, hold,
   hold at bay, hold back, hold fast, hold in, hold in check,
   hold in leash, hold up, impede, inhibit, inhibition, injunction,
   interdict, keep, keep back, keep from, keep in, keep in check,
   keep under control, lay under restraint, leadership,
   legal restraint, let down, let up, limit, limitation, lose ground,
   lose momentum, lose speed, moderate, monopoly, obstruct, power,
   prohibit, prohibition, protection, protectionism,
   protective tariff, pull, pull in, rationing, reef, rein in, reins,
   relax, repress, restrain, restraint, restraint of trade, restrict,
   retard, retardation, retrench, retrenchment, rule, running,
   self-control, set back, simmer down, slack off, slack up, slacken,
   slow, slow down, slow up, slowing down, smother, snub, stay,
   straiten, suppress, take in sail, tariff wall, thought control,
   throttle down, tiller, withhold