(1) one object, one body, (2) what on earth?, really?, (3) generally
Example sentences
彼が病気だというのは一体本当だろうか。
Can it be true that he is ill?
鉄道の道筋を決めるのは一体何だろうか。
What is it that determines the route a railway takes?
常識のある人なら、一体誰がそんなことを信じようか。
Who that has common sense can believe it?