A hollow cylinder is a three-dimensional geometric shape that has two circular bases and a curved surface connecting them. Unlike a solid cylinder, which is filled with material, a hollow cylinder has an empty space in the center. This shape is commonly used in various applications, such as pipes, tubes, and containers. The dimensions of a hollow cylinder are typically defined by its outer radius, inner radius, and height. The volume of a hollow cylinder can be calculated by subtracting the volume of the inner cylinder from the volume of the outer cylinder. This shape is important in engineering and architecture, where it is often used in structures and mechanical components.