Over 700 finance terms, Shmooped to perfection.
This is municipal debt which was incurred in connection with some asset that benefits more than one municipality's citizens and which is a joint obligation of more than one municipality. An example would be a public park with a border between two cities running through it, and which is used by citizens of both cities. (Imagine Minneapolis-St. Paul, except there's a park instead of the Mississippi River between them.) If they issued debt to pay for upkeep in the park, the debt would be coterminous in that it would be owed by the citizens of both cities.