guard

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

• intransitive verb To watch over to keep from being harmed or injured: synonym: defend.
• intransitive verb To supervise the entry and exit through; keep watch at.
• intransitive verb To watch over so as to prevent escape or violence.
• intransitive verb To keep from risk or curtailment; ensure the safety or integrity of.
• intransitive verb Sports To keep (an opposing player) from scoring or playing effectively, usually by remaining close to the player to disrupt offensive play.
• intransitive verb To maintain control over, as to prevent indiscretion.
• intransitive verb To furnish (a device or object) with a part that protects people from harm or injury.
• intransitive verb Archaic To escort as a guard.
• intransitive verb To take precautions.
• intransitive verb To serve as a guard.
• noun One who protects, keeps watch, or acts as a sentinel.
• noun One who supervises prisoners.
• noun An honor guard.
• noun Chiefly British A railway employee in charge of a train.
• noun Football One of the two offensive linemen on either side of the center.
• noun Basketball Either of the two players normally positioned in the backcourt who are responsible for bringing the ball to and initiating offensive plays from the frontcourt.
• noun Sports A defensive position or stance, as in boxing or fencing.
• noun The act or duty of guarding.
• noun Protection; watch.
• noun Something that gives protection; a safeguard.
• noun A device or attachment that prevents injury, damage, or loss, especially.
• noun An attachment or covering put on a machine to protect the operator or a part of the machine.
• noun A device on a foil, sword, or knife that protects the hand.
• noun A padded covering worn to protect a body part from injury.
• noun A small chain or band attached to a watch or bracelet to prevent loss.
• noun A ring worn to prevent a more valuable ring from sliding off the finger.
• noun Electronics A signal that prevents accidental activation of a device or ambiguous interpretation of data.
• idiom (off (one's) guard) Not alert; unprepared.
• idiom (on (one's) guard) Alert and watchful; cautious.
• idiom (stand guard) To keep watch.
• idiom (stand guard) To act as a sentinel.

from The Century Dictionary.

• noun A state of readiness to oppose attack; a state of defense; in general, a state of protection against injury or impairment of any kind.
• noun Specifically, a state of caution or vigilance; attentive observation designed to prevent surprise or attack; watch; heed: as, to keep guard; to be on one's guard; to keep a careful guard over the tongue.
• noun One who or that which protects or keeps in safety; one who or that which secures against danger, attack, loss, or injury; one who keeps protecting watch.
• noun Specifically— A man or body of men occupied in preserving a person or place from attack or injury, or in preventing an escape; he or they whose business it is to defend, or to prevent attack or surprise: as, a body-guard; a prison guard.
• noun Anything that keeps off evil: as, modesty is the guard of innocence.
• noun That which secures against hostile criticism or censure; a protection against malevolent or ignorant attacks upon one's reputation, opinions, etc.
• noun In fencing or boxing, a posture of passive defense; the arms or weapon in such a posture: as, to beat down one's guard.
• noun In the game of cricket, the position of the bat for most effectually defending the wicket.
• noun In Great Britain, a person who has charge of a mail-coach or a railway-train; a conductor; in the United States, a brakeman or gatekeeper on an elevated railroad.
• noun plural In cricket, the pads or protectors worn on the legs to prevent injury from swiftly thrown balls.
• noun Any part, appliance, or attachment designed or serving to protect or secure against harmful contact, injury, loss, or detriment of any kind.
• noun A chain or cord for fastening a watch, brooch, or bracelet to the dress of the wearer.
• noun Nautical, the railing of the promenade-deck of a steamer, intended to prevent persons from falling overboard; also, a widening of the deck of a side-wheel steamer by a framework of strong timbers which curve out on each side to the paddle-wheels, and protect them against collision with wharfs and boats.
• noun A metal frame placed over a nut in an engine, to prevent it from being jarred off.
• noun One of the fingers in a harvester in which the knives of the cutter-bar move.
• noun In bookbinding: A reinforcing slip placed between the leaves of a blank book designed for an album or a scrap-book. A narrow strip or narrow strips of paper sewed near the back of a book, made for inserted plates, with intent to keep the book flat, and prevent it from being thicker at the fore edge than at the back.
• noun A tide-lock between a dock and a river.