Cylinder, in geometry, is a solid figure with two identical bases that lie on parallel planes. Each base is bounded by a curved edge, called the directrix. The lateral surface (side) of a cylinder consists of parallel lines that join corresponding points on each base. When the directrixes of a cylinder are circles, the figure is called a circular cylinder. A right circular cylinder is a circular cylinder with a lateral surface that is perpendicular to the bases. Circular cylinders with a lateral surface not perpendicular to the bases are called oblique circular cylinders. A cylinder whose directrixes are ellipses is called a cylindroid.