Medical Definition of barrier
1a: a material object or set of objects that separates, demarcates, or serves as a barricade—see blood-brain barrier, placental barrierb: a contraceptive device (such as a condom or cervical cap) that prevents sperm from entering the uterus—often used before another noun <barrier contraception>
2: a factor that tends to restrict the free movement, mingling, or interbreeding of individuals or populations <behavioral and geographic barriers to hybridization>
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