Calculus is a way of using math to study changes in a system. Moral calculus, on the other hand, is a way of measuring morals and ethics, like choosing the lesser of two evils.
After an apple bonked him on the head, Newton used calculus to fine-tune his Law of Gravity. Don’t break that law, or you’ll fall right off the planet. Today, if you take a calculus class, you’ve probably already taken geometry and trigonometry. Well done. You’ll measure changes between infinitesimally small points or the area of a curve. In the medical world, a calculus is a hard, crusty mass like a kidney stone; or tartar on a tooth. In Latin, calculus means "small pebble used for counting."