A swan neck duct is a jet engine component, usually situated within the compression or turbine expansion system.
[1] Many jet engines have a short annular passageway, linking two components, where there is a large change in mean radius, from front to rear.
The shape of the resulting passageway is often similar to that of a swan or goose neck.
Consequently, such a passageway is called a swan (or goose) neck duct.
For the latter purpose a swan neck duct is an example of an inter-turbine diffuser.