An endobronchial blocker (also called a bronchial blocker) is a medical device used to isolate one lung during surgery or other procedures, allowing for one-lung ventilation. It's a flexible tube, often with a balloon at its distal end, that is inserted into a main bronchus (either right or left) to selectively block airflow to that lung. This allows surgeons to operate on the isolated lung without interference from the other lung.