This term denotes Nikon's top-end range of optically superior lenses. If you see an 'S' in a Nikon lens' name, that means it's part of the S-Line, and is designed to offer pin-sharp resolution, with (according to Nikon) 'superb point-image reproduction, even at the widest apertures'. They're designed to reproduce point light sources more accurately, and provide aesthetically pleasing bokeh characteristics. As such, they tend to be the more expensive members of the line-up.