Math is the science of numbers. You may start out learning addition and subtraction in math, and then end up years later tackling multivariable implicit differentiation problems. Say what?!
Math is the common, shortened form of mathematics, which comes from the Greek mathematike tekhne, "mathematical science," from the root mathema, "knowledge or science." Math is one of the basic subjects that are covered in nearly every school, from basic addition to calculus. If someone says, "Do the math," they usually mean, "Just think about it — it's so obvious."