PHIGS is an acronym for Programmers Hierarchical Interactive Graphics System.
It is an API for rendering 2D and 3D computer graphics. It allowed the programmer to develop a hierarchical data structure describing the scene to be drawn. This would typically include definitions of all the polygons, lines and points as well as their various attributes, such as colour, line width and so on. The structure would then be processed in a single operation. This is known as deferred rendering, as opposed to immediate rendering (where the calls to the graphics API cause drawing to occur immediately).
PHIGS originally lacked the capability to render illuminated scenes, and was superceded by PHIGS+. PHIGS+ worksed in essentially the same manner, but added methods for lighting a 3D scene, and was often referred to as PHIGS PLUS (where the PLUS was a somewhat contrived acronym for "Plus Lumiere Und Shading"). PHIGS+ also introduced more advanced graphics primitives, such as NURBS surfaces.