I will bypass writing 50 pages on the various definitions of CAD, the acronym. Here let's consider what we mean when we say ``computer aided design'' itself. I propose the following definition:
Computer aided design is the modeling of physical systems on computers, allowing both interactive and automatic analysis of design variants, and the expression of designs in a form suitable for manufacturing.
I think that this definition encompasses all of the types of work that is subsumed under the CAD umbrella, in all the various areas of application.
This definition implies that simulation is a far more important part of CAD than design description. I believe that this is true. Also, computer graphics has nothing at all to do with CAD, except as the servant of design, simulation, or presentation.