Merriam-Webster Collegiate Dictionary - barrier

 noun  Etymology: Middle English barrere, from Anglo-French, from barre bar  Date: 14th century  1.  a. something material that blocks or is intended to block passage highway ~s a ~ contraceptive  b. a natural formation or structure that prevents or hinders movement or action geographic ~s to species dissemination ~ beaches drugs that cross the placental ~  2. plural, often capitalized a medieval war game in which combatants fight on foot with a fence or railing between them  3. something immaterial that impedes or separates ; obstacle behavioral ~s trade ~s