1. A structure, such as a fence, built to bar passage.
2. something immaterial that obstructs or impedes: intolerance is a barrier to understanding. see synonyms at obstacle.
3. physiology A membrane, tissue, or mechanism that blocks the passage of certain substances.
4. ecology A physical or biological factor that limits the migration, interbreeding, or free movement of individuals or populations.
5. A boundary or limit.
6. something that separates or holds apart.
7. A movable gate that keeps racehorses in line before the start of a race.
8. the palisades or fences enclosing the lists of a medieval tournament. often used in the plural.
9. geology an ice barrier.

1. A structure that bars passage.
2. an obstacle or impediment.
3. A boundary or limit.

1. A carpentry obstruction, stockade, or other obstacle made in a passage in order to stop an enemy.
2. A fortress or fortified town, on the frontier of a country, commanding an avenue of approach.
3. A fence or railing to mark the limits of a place, or to keep back a crowd.
4. any obstruction; anything which hinders approach or attack.
5. any limit or boundary; a line of separation.

1. to shut in or off with a barrier.
2. in fortification, anything, as a palisade or stockade, designed to obstruct entrance into a fortified place.
3. 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).
4. 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.
5. A fortress or fortified town on the frontier of a country.
6. A limit or boundary of any kind; a line of separation.
7. the gate, in towns on the continent of europe, at which local revenue duties are collected.
8. in china, a subordinate customs station placed on an inland trade-route for the collection of duties on goods in transit.
9. 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.
10. 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.
11. in phytogeography, any obstacle which prevents or restricts the migration of plants.
12. see the qualifying words.
13. A treaty in 1748, terminating the war of the austrian succession.
14. in 1831, for the settlement of the belgian question
15. in 1840, for the settlement of the relations between turkey and egypt
16. in 1871, abrogating the neutrality of the black sea.
17. A treaty between prussia and austria in 1866, by which the former power succeeded the latter in the hegemony of germany.
18. that of 1809, between france and austria, in favor of the former
19. that of 1815, by the congress of the european states, reorganizing the affairs of europe
20. that of 1864, between denmark and allied austria and prussia, ending the Schleswig-holstein war
21. that of 1866, between austria and italy, by which venetia was ceded to the latter.

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