Its non-literal meaning is the area of thought or opinion in between the two absolutes of right and wrong or good and bad. Normals, mundanes, induhviduals, fundamentalists, and religious fanatics are all people who tend to choose one of the two absolutes and pretend that the shades of grey don't exist. This tends to hamper their mental growth while giving them an excuse to try and force everyone else to either choose the good or right thing, or automatically be bad or wrong.