A prism is a solid with bases that are polygons and the sides are flat surfaces. Strictly speaking, a cylinder is not a prism.

Strictly speaking, a cylinder is not a prism. Howeve,r it is extremely similar. If you imagine a prism with regular polygons for bases,

as you increase the number of sides, the solid gets to look just like a cylinder.

So we can say that a cylinder is a prism with an infinite number of faces.