A cross section is what you see when you slice straight through something and look at the exposed surface. Imagine cutting an apple in half from top to bottom: the flat surface you see inside, with the star-shaped seed pattern, is a cross section of the apple. If you cut it the other way, horizontally through the middle, you'd see a different cross section showing a ring of seeds.

Scientists and engineers use cross sections to understand how things work inside. A cross section of a tree trunk reveals its growth rings. A cross section of the Earth shows its layers: crust, mantle, and core. Architects draw cross sections of buildings to show what's behind the walls, where the pipes run, and how the floors connect.

The term also describes a representative sample of a larger group. If a teacher surveys a cross section of students about lunch preferences, she asks students from different grades, backgrounds, and friend groups to get a complete picture of what the whole school thinks. This kind of cross section captures the full range of a group, just as slicing through an object reveals what it's made of.