Portal is a mix between a computer novel and an interactive game. It was published for the Amiga in 1986 by Activision, written by Rob Swigart, produced by Brad Fregger and programmed by Nexa Corporation. Versions for the Macintosh, Commodore 64, Apple II, and the IBM PC were later released. A unique game for its time, Portal was one part text-driven adventure (à la Zork or Planetfall) but with a graphical interface.
Source: Wikipedia, "Portal (interactive novel)", available under the CC-BY-SA License.