What is one definition of the central or main idea? a summary of a piece of writing a one-on-one or small group question-and-answer session the thesis statement of original documents an object created by someone with firsthand experience

One definition of the central or main idea is a summary of a piece of writing.

The central idea is the most important or main thought of a piece of writing (E.g.: A text, paragraph, story, etc), and tells the reader what the text is about.

It is also usually defined as a short summary of the piece of writing since it presents a general idea that compasses the other ideas of the text by reflecting emotions, conflicts and relevant experiences of the main characters of a story, for example.  

from the excerpt from the "the dark game" the reader can infer that folke cronholm was spying for the allied nations.