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." [1913 Webster]

2.  Hence, an instrument or means of curbing, restraining, or governing; government; restraint. Milton. [1913 Webster]

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." [1913 Webster]

2.  To restrain; to control; to check. [1913 Webster]
"Being once chafed, he can not
Be reined again to temperance." [1913 Webster]

   To be guided by reins. Shak. [1913 Webster]

rein, n. & v.
--n. (in sing. or pl.)
1 a long narrow strap with each end attached to the bit, used to guide or check a horse etc. in riding or driving.
2 a similar device used to restrain a young child.
3 a means of control.
--v.tr.
1 check or manage with reins.
2 (foll. by up, back) pull up or back with reins.
3 (foll. by in) hold in as with reins; restrain.
4 govern, restrain, control.

Idiom:
draw rein
1 stop one's horse.
2 pull up.
3 abandon an effort. give free rein to remove constraints from; allow full scope to. keep a tight rein on allow little freedom to.

Derivative:
reinless adj.

Etymology:
ME f. OF rene, reigne, earlier resne, ult. f. L retinere RETAIN

N  connection, vinculum, link, connective, connection, junction, bond of union, copula, hyphen, intermedium, bracket, bridge, stepping-stone, isthmus, bond, tendon, tendril, fiber, cord, cordage, riband, ribbon, rope, guy, cable, line, halser, hawser, painter, moorings, wire, chain, string, fastener, fastening, tie, ligament, ligature, strap, tackle, rigging, standing rigging, running rigging, traces, harness, yoke, band ribband, bandage, brace, roller, fillet, inkle, with, withe, withy, thong, braid, girder, tiebeam, girth, girdle, cestus, garter, halter, noose, lasso, surcingle, knot, running knot, cabestro, cinch, lariat, legadero, oxreim, suspenders, pin, corking pin, nail, brad, tack, skewer, staple, corrugated fastener, clamp, U-clamp, C-clamp, cramp, cramp iron, ratchet, detent, larigo, pawl, terret, treenail, screw, button, buckle, clasp, hasp, hinge, hank, catch, latch, bolt, latchet, tag, tooth, hook, hook and eye, lock, holdfast, padlock, rivet, anchor, grappling iron, trennel, stake, post, cement, glue, gum, paste, size, wafer, solder, lute, putty, birdlime, mortar, stucco, plaster, grout, viscum, shackle, rein, prop.

N  prison, prison house, jail, gaol, cage, coop, den, cell, stronghold, fortress, keep, donjon, dungeon, Bastille, oubliette, bridewell, house of correction, hulks, tollbooth, panopticon, penitentiary, guardroom, lockup, hold, round house, watch house, station house, sponging house, station, house of detention, black hole, pen, fold, pound, inclosure, isolation (exclusion), penal settlement, penal colony, bilboes, stocks, limbo, quod, calaboose, chauki, choky, thana, workhouse, Newgate, Fleet, Marshalsea, King's Bench, Queen's Bench, bond, bandage, irons, pinion, gyve, fetter, shackle, trammel, manacle, handcuff, straight jacket, strait jacket, strait-jacket, strait-waistcoat, hopples, vice, vise, yoke, collar, halter, harness, muzzle, gag, bit, brake, curb, snaffle, bridle, rein, reins, bearing rein, martingale, leading string, tether, picket, band, guy, chain, cord, cavesson, hackamore, headstall, jaquima, lines, ribbons, bolt, deadbolt, bar, lock, police lock, combination lock, padlock, rail, wall, stone wall, paling, palisade, fence, picket fence, barbed wire fence, Cyclone fence, stockade fence, chain-link fence, barrier, barricade, drag.

alaska rein orchid, bearing rein, bog rein orchid, coastal rein orchid, draw rein, free rein, leading rein, rein in, rein orchid, rein orchis, round-leaved rein orchid