What type of bus has a hinged section in the middle?

A bus with a hinged section in the middle is called an articulated bus.

An articulated bus is built from two or more rigid sections connected by a pivoting joint. A flexible bellows-like cover protects the joint and lets the vehicle bend while turning. This arrangement allows a bus to be considerably longer than a conventional rigid bus while remaining maneuverable on city streets.

Most articulated buses are single-deck vehicles and are commonly about 18 meters long, compared with roughly 11 to 14 meters for many standard rigid buses. Their extra length provides more passenger capacity, while multiple doors can speed boarding and unloading. They are therefore especially common on busy urban routes and bus rapid transit systems.

The terms “bendy bus,” “accordion bus,” “slinky bus,” and “artic” are informal names for the same basic vehicle type. A trolleybus can also be articulated, because trolleybus describes its electrical power system rather than its body structure. A double-decker, by contrast, has two passenger levels and does not necessarily have a hinged middle section.