Something that blocks a path, a pipe, or a process so that things cannot move through normally — like a fallen tree blocking a road, or a blood clot causing an obstruction in a vein. It can also mean the act of deliberately stopping something from happening, such as 'obstruction of justice,' when someone tries to stop the police or a court from doing their job. In sports like basketball or hockey, 'obstruction' is a foul for blocking another player's way unfairly.