A frustum is a three-dimensional geometric shape that results from slicing the top off a cone or a pyramid. It has two parallel circular or polygonal bases, with the top base being smaller than the bottom base. The sides of a frustum are typically slanted, connecting the edges of the two bases. Frustums are commonly found in various real-world objects, such as cups, lampshades, and certain architectural structures. They are important in fields like mathematics and engineering, where calculating volume and surface area is essential for design and construction purposes.