Definition of "unless"

Secondly, When a body is once in motion it will continue to move forever, unless something stops it. When a ball is struck on the surface of the earth, the friction of the earth and the resistance of the air soon stop its motion.

1839, Denison Olmsted, A Compendium of Astronomy, page 95

Unless someone like you...cares a whole awful lot...nothing is going to get better...It's not.

1971, Dr. Seuss, The Lorax

Who would know of Alexander or Caesar, or would respect the Stoics or the Peripatetics, unless they had been distinguished by the memorials of writers?

1159, John of Salisbury, Policraticus, edited by Cary J. Nederman, Cambridge University Press, 1990, page 3 (Google Books view)

And Joab said, As God liveth, unless thou hadst spoken, surely then in the morning the people had gone up every one from following his brother.

1611, The Holy Bible, […] (King James Version), London: […] Robert Barker, […], 2 Samuel 2:27

Unless He were omnipotent, we could not be sure of His ability to bless us.

1867, William Robinson Pirie, Natural Theology: An Inquiry Into the Fundamental Principles of Religious, Moral, and Political Science, page 75 (Google Books view)

Q. If Lieutenant P. had given the word "Fire," would you have fired, and at what?—A. I should not have known, unless he had told me what to fire at.

1809, The Naval Chronicle for 1809: Containing a General and Biographical History of the Royal Navy of the United Kingdom with a Variety of Original Papers on Nautical Subjects. Vol. XXII. (from July to December), Cambridge University Press, 2010, page 482 (Google Books view)

Lie down and finish out the night, since you are here; but, for heaven's sake! don't repeat that horrid noise: nothing could excuse it, unless you were having your throat cut!

1847, Emily Brontë, chapter III, in Wuthering Heights

Suppose Breugel had done the whole painting with no legs. Then, titled as it is, it would be mystifying, unless someone were to say: the boy has fallen in the waters and they have closed over him, calm is restored, life goes on (as in The Israelites Crossing the Red Sea).

1981, Arthur C. Danto, The Transfiguration of the Commonplace: A Philosophy of Art, page 118 (Google Books view)