debouch(noun, verb) noun1. (geography) A narrow outlet from which a body of water pours.2. (military) A fortress at the end of a defile. verb1. march out (as from a defile) into open ground- The regiments debouched from the valleyDefinition categories: motion, march, process2. pass out or emerge; especially of rivers- The tributary debouched into the big riverDefinition categories: change, egress, emerge, issue