rein

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of two long narrow straps attached to each end of the bit of a bridle and used by a rider or driver to control a horse or other animal.
• noun A means of restraining or checking.
• noun A means of controlling or directing.
• intransitive verb To check or hold back by the use of reins. Often used with in or up.
• intransitive verb To restrain or control. Often used with in.
• intransitive verb To control a horse, for example, with reins. Often used with in or up.
• idiom (draw rein) To stop a horse, for example, by pulling on the reins.
• idiom (draw in the reins) To slow down or stop a horse or other animal by putting pressure on the reins.
• idiom (draw in the reins) To restrain or control.
• idiom (free/full) To release from restraints; allow to go unchecked.

from The Century Dictionary.

• To govern, guide, or restrain by reins or a bridle.
• To restrain; control.
• To carry stiffly, as a horse does its head or neck under a bearing-rein.
• To obey the reins.
• noun An obsolete singular of reins.
• noun The strap of a bridle, fastened to the curb or snaffle on each side, by which the rider or driver restrains and guides the animal driven; any thong or cord used for the same purpose. See cut under harness.
• noun A rope of twisted and greased rawhide.
• noun plural The handles of blacksmiths' tongs, on which the ring or coupler slides.
• noun Figuratively, any means of curbing, restraining, or governing; government; restraint.

from the GNU version of the Collaborative International Dictionary of English.

• noun The strap of a bridle, fastened to the curb or snaffle on each side, by which the rider or driver governs the horse.
• noun Hence, an instrument or means of curbing, restraining, or governing; government; restraint.
• noun to give license to; to leave withouut restrain.
• noun to take the guidance or government; to assume control.
• intransitive verb rare To be guided by reins.
• transitive verb To govern or direct with the reins.
• transitive verb To restrain; to control; to check.
• transitive verb 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun archaic, chiefly in plural A kidney.
• noun A strap or rope attached to the bridle or bit, used to control a horse, animal or young child.
• verb To direct or stop a horse by using reins.
• verb this sense?) (usually "rein in") To stop or restrain a horse. Also used figuratively

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun one of a pair of long straps (usually connected to the bit or the headpiece) used to control a horse
• verb control and direct with or as if by reins
• noun any means of control
• verb keep in check
• verb stop or check by or as if by a pull at the reins
• verb stop or slow up one's horse or oneself by or as if by pulling the reins