CAD software, also referred to as Computer Aided Design (Computer Aided Drafting?) software and in the past as computer aided drafting software, refers to software programs that assist engineers and designers in a wide variety of industries to design and manufacture physical products ranging from buildings, bridges, roads, aircraft, ships and cars to digital cameras, mobile phones, TVs, clothes and of course computers! CAD software is often referred to as CAD CAM software ('CAM' is the acronym for Computer Aided Machining or Computer Aided Manufacturing). (A somewhat related concept is CIM (Computer Integrated Manufacturing).)While he could never have foreseen today's CAD software, no CAD software history would be complete unless it started with the mathematician Euclid of Alexandria, who, in his 350 B.C. treatise on mathematics "The Elements" expounded many of the postulates and axioms that are the foundations of the Euclidian geometry upon which today's CAD software systems are built.It was more than 2,300 years after Euclid that the first true CAD software, a very innovative system (although of course primitive compared to today's CAD software) called "Sketchpad"  was developed by Ivan Sutherland as part of his PhD thesis at MIT in the early 1960s. Sketchpad was especially innovative CAD software because the designer interacted with the computer graphically by using a light pen to draw on the computer's monitor. It is a tribute to Ivan Sutherland's ingenuity that even in 2004, when operations which took hours on 1960s computer technology can be executed in less than a millionth of a second and touch-sensitive TFT combination display/input devices are readily available, there is no leading CAD software that has yet incorporated such directness into its user interface.In 1987 John Ousterhout devised Tcl as an interface to ease research he was then doing with CAD software for IC and PCB design. Tcl has become highly popular in this area, see for example this site .The Autodesk/Autocad history makes a fantastic read  as does this comprehensive 1970s - 2000 CAD software  history.
Software such as Ousterhout worked on is sometimes called "ECAD" (Electronics CAD) or EDA (Electronic Design Automation).