A cylinder is one of the most basic curvilinear geometric shapes, the formed by the points at a fixed distance from a given straight line, the axis of the cylinder. The solid enclosed by this surface and by two planes perpendicular to the axis is also called a cylinder. The and the of a cylinder have been known since deep antiquity.

In , a cylinder is defined more broadly as any spanned by a one-parameter family of parallel lines. A cylinder whose is an , , or is called an elliptic cylinder, parabolic cylinder, or hyperbolic cylinder.