1.  barrier(n = noun.artifact) - a structure or object that impedes free movement;
is a kind of impediment, impedimenta, obstructer, obstruction, obstructor
has particulars: balusters, balustrade, banister, bannister, handrail, barricade, breakwater, bulwark, groin, groyne, jetty, mole, seawall, bulwark, crash barrier, dam, dike, dyke, fence, fencing, fender, wing, grate, grating, hurdle, movable barrier, rail, railing, revetment, barricade, roadblock, starting gate, starting stall

2.  barrier(n = noun.cognition) roadblock - any condition that makes it difficult to make progress or to achieve an objective; "intolerance is a barrier to understanding"
is a kind of obstacle, obstruction
has particulars: ideological barrier, language barrier

3.  barrier(n = noun.object) - anything serving to maintain separation by obstructing vision or access;
is a kind of mechanism
has particulars: bar, blood-brain barrier, curtain

1.  A carpentry obstruction, stockade, or other obstacle made in a passage in order to stop an enemy. [1913 Webster]

2.  A fortress or fortified town, on the frontier of a country, commanding an avenue of approach. [1913 Webster]

3.  A fence or railing to mark the limits of a place, or to keep back a crowd. [1913 Webster]
"No sooner were the barriers opened, than he paced into the lists." [1913 Webster]

4.  Any obstruction; anything which hinders approach or attack. Hopkinson. [1913 Webster]

5.  Any limit or boundary; a line of separation. [1913 Webster]
"'Twixt that [instinct] and reason, what a nice barrier!" [1913 Webster]

barrier, n.
1 a fence or other obstacle that bars advance or access.
2 an obstacle or circumstance that keeps people or things apart, or prevents communication (class barriers; a language barrier).
3 anything that prevents progress or success.
4 a gate at a car park, railway station, etc., that controls access.
5 colloq. = sound barrier.

Idiom:
barrier cream a cream used to protect the skin from damage or infection. barrier reef a coral reef separated from the shore by a broad deep channel.

Etymology:
ME f. AF barrere, OF barriere

N  refuge, sanctuary, retreat, fastness, acropolis, keep, last resort, ward, prison, asylum, ark, home, refuge for the destitute, almshouse, hiding place, sanctum sanctorum, roadstead, anchorage, breakwater, mole, port, haven, harbor, harbor of refuge, seaport, pier, jetty, embankment, quay, covert, cover, shelter, screen, lee wall, wing, shield, umbrella, barrier, dashboard, dasher, wall, fort, anchor, kedge, grapnel, grappling iron, sheet anchor, killick, mainstay, support, cheek, ballast, jury mast, vent-peg, safety valve, blow-off valve, safety lamp, lightning rod, lightning conductor, safety belt, airbag, seat belt, antilock brakes, antiskid tires, snow tires, means of escape, lifeboat, lifejacket, life buoy, swimming belt, cork jacket, parachute, plank, steppingstone, emergency landing, safeguard, any port in a storm, bibere venenum in auro, valet anchora virtus.

N  hindrance, prevention, preclusion, obstruction, stoppage, embolus, embolism, infarct, interruption, interception, interclusion, hindrance, impedition, retardment, retardation, embarrassment, oppilation, coarctation, stricture, restriction, restraint, inhibition, blockade, interference, interposition, obtrusion, discouragement, discountenance, impediment, let, obstacle, obstruction, knot, knag, check, hitch, contretemps, screw loose, grit in the oil, bar, stile, barrier, turnstile, turnpike, gate, portcullis, beaver dam, trocha, barricade, wall, dead wall, sea wall, levee breakwater, groyne, bulkhead, block, buffer, stopper, boom, dam, weir, burrock, drawback, objection, stumbling-block, stumbling-stone, lion in the path, snag, snags and sawyers, encumbrance, incumbrance, clog, skid, shoe, spoke, drag, drag chain, drag weight, stay, stop, preventive, prophylactic, load, burden, fardel, onus, millstone round one's neck, impedimenta, dead weight, lumber, pack, nightmare, Ephialtes, incubus, old man of the sea, remora, difficulty, insuperable, obstacle, estoppel, ill wind, head wind, trammel, tether, hold back, counterpoise, damper, wet blanket, hinderer, marplot, killjoy, party pooper, party crasher, interloper, trail of a red herring, opponent, hindering, obstructive, obstruent, impeditive, impedient, intercipient, prophylactic, impedimentary, in the way of, unfavorable, onerous, burdensome, cumbrous, cumbersome, obtrusive, hindered, windbound, waterlogged, heavy laden, hard pressed, unassisted &c (assist), single-handed, alone, deserted, occurrent nubes.

N  defense, protection, guard, ward, shielding, propugnation, preservation, guardianship, area defense, site defense, self-defense, self-preservation, resistance, safeguard, balistraria, bunker, screen, camouflage, fortification, munition, muniment, trench, foxhole, bulwark, fosse, moat, ditch, entrenchment, intrenchment, kila, dike, dyke, parapet, sunk fence, embankment, mound, mole, bank, sandbag, revetment, earth work, field- work, fence, wall dead wall, contravallation, paling, palisade, haha, stockade, stoccado, laager, sangar, barrier, barricade, boom, portcullis, chevaux de frise, abatis, abattis, abbatis, vallum, circumvallation, battlement, rampart, scarp, escarp, counter-scarp, glacis, casemate, vallation, vanfos, buttress, abutment, shore, breastwork, banquette, curtain, mantlet, bastion, redan, ravelin, vauntmure, advance work, horn work, outwork, barbacan, barbican, redoubt, fort-elage, fort-alice, lines, loophole, machicolation, sally port, hold, stronghold, fastness, asylum, keep, donjon, dungeon, fortress, citadel, capitol, castle, tower of strength, tower of strength, fort, barracoon, pah, sconce, martello tower, peelhouse, blockhouse, rath, wooden walls, bulletproof vest, armored vest, buffer, corner stone, fender, apron, mask, gauntlet, thimble, carapace, armor, shield, buckler, aegis, breastplate, backplate, cowcatcher, face guard, scutum, cuirass, habergeon, mail, coat of mail, brigandine, hauberk, lorication, helmet, helm, bassinet, salade, heaume, morion, murrion, armet, cabaset, vizor, casquetel, siege cap, headpiece, casque, pickelhaube, vambrace, shako, bearskin, panoply, truncheon, garrison, picket, piquet, defender, protector, guardian, bodyguard, champion, knight-errant, Paladin, propugner, bulletproof window, hardened site, defending, defensive, mural, armed, armed at all points, armed cap-a-pie, armed to the teeth, panoplied, iron-plated, ironclad, loopholed, castellated, machicolated, casemated, defended, proof against, armored, ballproof, bulletproof, hardened, defensively, on the defense, on the defensive, in defense, at bay, pro aris et focis, Int, no surrender!, defense not defiance, Dieu defend le droit, fidei defensor, defender of the faith.

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.

N  inclosure, envelope, case, wrapper, girdle, pen, fold, pen fold, in fold, sheep fold, paddock, pound, corral, yard, net, seine net, wall, hedge, hedge row, espalier, fence, pale, paling, balustrade, rail, railing, quickset hedge, park paling, circumvallation, enceinte, ring fence, barrier, barricade, gate, gateway, bent, dingle, door, hatch, cordon, prison, dike, dyke, ditch, fosse, moat.

barrier island, barrier reef, barrier strip, blood-brain barrier, crash barrier, great barrier reef, heat barrier, ideological barrier, import barrier, junction barrier, language barrier, movable barrier, sonic barrier, sound barrier, thermal barrier, trade barrier