An Immelmann is an inversion named after an aircraft maneuver used by World War I pilots. The inversion begins with the first half of a vertical loop. As the top of the loop is approached, the train is fully inverted. The train continues heading away from the half loop as it becomes right-side up, thus the inversion is exited in the opposite direction to which it is entered.
An Immelmann travelled in reverse is a dive loop.