Get Specular Colour
The input connection is not used by this node (consistent with all Get nodes).
Specular Colour is the additive component of the appearance of a surface which was reflected from light sources by an amount that strongly depended on the viewing angle, surface normal and light direction. Usually this comes from "specular highlights" settings in the GUI. It is not the specularity of the surface, but the final specular contribution at that point. Currently, ray traced reflections and transmissions are not stored in the specular component, but this may change in future.
Graphical User Interface, a general term that refers to the interface of any program running in a modern graphical operating system and which does not operate exclusively from the commandline.