A program that allowed users to draw on a screen with a light pen. It supported constraints (e.g. drawing a constrained ellipse produced a circle). It also had some computer aided design features (e.g. computing loads on beams).
Sketchpad was the subject of Ivan E. Sutherland's 1963 MIT PhD thesis, which opened the field of computer graphics. It was the progenitor of computer drawing packages like MacDraw or Adobe Illustrator. There is a film of Sketchpad in action.
[Sammet 1969, p. 678].