Abbreviation: N = "Nano Crystal Coat" (Nikon)

The abbreviations means "Nano Crystal Coat"

Explanation from Nikon

Nano Crystal Coat is an antireflective coating that originated in the development of NSR-series (Nikon Step and Repeat) semiconductor manufacturing devices. It virtually eliminates internal lens element reflections across a wide range of wavelengths, and is particularly effective in reducing ghost and flare peculiar to ultra-wideangle lenses. Nano Crystal Coat employs multiple layers of Nikonís outstanding extra-low refractive index coating, which features ultra-fine crystallized particles of nano size (one nanometer equals one millionth of a mm). Nikon now proudly marks a world first by applying this coating technology to a wide range of lenses for use in consumer optical products.

See examples of effected products

Nikon AF-S 14-24mm f/2,8 G ED
Nikon AF-S 24-70mm f/2,8 G ED
Nikon AF-S 70-200mm f/2,8 G IF-ED VR II