[TA]Avoid the incorrect term left main stem (or mainstem) bronchus. primary division of the tracheobronchial tree arising as the left branch of the bifurcation of the trachea, then passing in front of the esophagus and enters the hilum of the left lung where it divides into a superior lobe bronchus and an inferior lobe bronchus. It is longer, of narrower caliber, and more nearly horizontal than the right main bronchus, hence, aspirated objects enter it less frequently. SYN: bronchus principalis sinister [TA].