A cross-section is a 2-dimensional “slice” of a 3-dimensional figure. Another way to think of it is finding the intersection of a 2-dimensional plane and a 3-dimensional figure.
For any given 3-dimensional figure, the cross-section depends on the orientation of the plane or “slice”. For example, consider a cylinder. Various cross sections might result in a circle or rectangle, as shown below, or any of several other shapes.
