The gist, the essence, the major parts of something — that's its outline.
The origins of outline come from the artistic practice of marking a line around the outer edge of a person or shape in a picture before filling it in. Only later, in the 1800s, did it come to be applied to verbal descriptions, too. Technically an outline suggests a condensed form of something, but as any high school or college student knows, course outlines have a habit of being pretty darn long. In 1919 H.G. Wells wrote a book called An Outline of History. Its length? 1,324 pages.