"DX" is Nikon’s name for the APS-C sensor size (24x16mm) DSLR system as opposed to the "FX" full frame cameras. The brand has a special series of DX lenses which have a smaller image projection area to cover the smaller sensor.

The Nikon DX mount is physically identical to the standard F mount, but the lenses are only interchangeable from FX to DX. Using an FX lens on a DX camera will work, but using DX lenses on FX bodies will result in heavy vignetting.