barrier

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

• noun A material formation or structure, such as a mountain range or wall, that prevents passage or access.
• noun Something immaterial that obstructs or impedes.
• noun Physiology A membrane, tissue, or mechanism that blocks the passage of certain substances.
• noun Ecology A physical or biological factor that limits the migration, interbreeding, or free movement of individuals or populations.
• noun A movable gate that keeps racehorses in line before the start of a race.
• noun The palisades or fences enclosing the lists of a medieval tournament.
• noun Geology An ice barrier.

from The Century Dictionary.

• noun In fortification, anything, as a palisade or stockade, designed to obstruct entrance into a fortified place.
• noun plural The palisades or railing surrounding the ground where tourneys and justs were carried on; hence, the sports themselves (formerly sometimes with the plural in a singular sense).
• noun Any obstruction; anything which hinders approach, attack, or progress; anything standing in the way; an obstacle: as, to build a wall as a barrier against trespassers; constitutional barriers.
• noun A fortress or fortified town on the frontier of a country.
• noun A limit or boundary of any kind; a line of separation.
• noun The gate, in towns on the continent of Europe, at which local revenue duties are collected.
• noun In China, a subordinate customs station placed on an inland trade-route for the collection of duties on goods in transit.
• noun In coal-mining, a solid block of coal left unworked between two collieries, for security against the accidents which might occur in consequence of communication between them.
• noun See the qualifying words.
• noun A treaty in 1748, terminating the War of the Austrian Succession.
• noun in 1831, for the settlement of the Belgian question
• noun in 1840, for the settlement of the relations between Turkey and Egypt
• noun in 1871, abrogating the neutrality of the Black Sea.
• noun A treaty between Prussia and Austria in 1866, by which the former power succeeded the latter in the hegemony of Germany.
• noun that of 1809, between France and Austria, in favor of the former
• noun that of 1815, by the congress of the European states, reorganizing the affairs of Europe
• noun that of 1864, between Denmark and allied Austria and Prussia, ending the Schleswig-Holstein war
• noun that of 1866, between Austria and Italy, by which Venetia was ceded to the latter.
• noun The starting apparatus used in races: designed to keep the horses behind a certain line until the word is given, and thus prevent unfair advantage in starting.
• noun In phytogeography, any obstacle which prevents or restricts the migration of plants.
• To shut in or off with a barrier.

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

• noun (Fort.) A carpentry obstruction, stockade, or other obstacle made in a passage in order to stop an enemy.
• noun A fortress or fortified town, on the frontier of a country, commanding an avenue of approach.
• noun A fence or railing to mark the limits of a place, or to keep back a crowd.
• noun Any obstruction; anything which hinders approach or attack.
• noun Any limit or boundary; a line of separation.
• noun a heavy gate to close the opening through a barrier.
• noun a form of coral reef which runs in the general direction of the shore, and incloses a lagoon channel more or less extensive.
• noun [Obs.] to fight with a barrier between, as a martial exercise.

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

• noun A structure that bars passage.
• noun An obstacle or impediment.
• noun A boundary or limit.

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

• noun a structure or object that impedes free movement
• noun any condition that makes it difficult to make progress or to achieve an objective
• noun anything serving to maintain separation by obstructing vision or access