Nikon F-mount teleconverter
The Nikon F-mount teleconverters are a group of magnifying lenses mounted between the lens and camera bodies using the Nikon F-mount. The 1.4×, 1.7× and 2× converters are available separately; a fourth, the 1.25×, is available only with Nikon's newest 800mm supertelephoto lens.
Source: Wikipedia — Nikon F-mount teleconverter (CC BY-SA 4.0)