A prism is a solid shape that has the same cross-section all the way through.
These three shapes are prisms.
(Sometimes the ‘cross-section’ is called the 'base'.)
The distance between the two ends of the prism is always called the height of the prism even if the prism is lying horizontally (like the triangular prism above).
These two shapes are not prisms.The cross-section changes at each level.
For any prism:
This shape is a triangular prism so the area of the cross-section is the area of a triangle.
Area of the triangle: