A multiple integral is a mathematical concept used to calculate the integral of functions with more than one variable. It extends the idea of a single integral, which finds the area under a curve, to higher dimensions. Common types include double integrals, which are used for functions of two variables, and triple integrals, for functions of three variables. These integrals help in finding volumes, areas, and other quantities in multi-dimensional spaces. In practical applications, multiple integrals are used in various fields such as physics, engineering, and economics. For example, they can determine the mass of an object with varying density or calculate probabilities in statistics. The process involves iteratively integrating each variable while keeping the others constant, often requiring specific techniques like Fubini's theorem for simplification.