cross section – the 2D section (or slice) created when a 3D shape is cut vertically or horizontally

To remember the meaning of cross section, use the following mnemonic:

It was a cross section (cross section) after being cut from the cube (3D shape).

The shape of a cross section depends on both the shape of the original object and the direction at which it is cut. For example, if you slice straight across a cylinder (like cutting a tin can horizontally), you get a circle. If you slice it vertically, you get a rectangle. 

Cross sections are useful in geometry for understanding the structure of 3D shapes, and they appear in many real world applications, from medical imaging (CT scans to show cross sections of the human body) to architecture and engineering, where understanding how a structure looks when "cut" at different points helps in design and analysis.