Provides the "Altitude" of the current Render State. The Render State is affected by any shaders which were called before this function is called, among other things. Get Altitude provides the signed difference between Get Position and Get Position in Geometry in the direction of Get Normal in Geometry. For a planet this basically means the altitude of the current point being rendered. Position is modified by displacement shaders, therefore so is altitude. Refer to Get Position for more information.
The input connection is not used by this node (consistent with all "Get" nodes).
Literally, to change the position of something. In graphics terminology to displace a surface is to modify its geometric (3D) structure using reference data of some kind. For example, a grayscale image might be taken as input, with black areas indicating no displacement of the surface, and white indicating maximum displacement. In Terragen 2 displacement is used to create all terrain by taking heightfield or procedural data as input and using it to displace the normally flat sphere of the planet.