cylinder, in mathematics, surface generated by a line moving parallel to a given fixed line and continually intersecting a given fixed curve called the directrix; each line of the family of lines forming the cylinder is called a ruling, or generator. If the directrix is a conic section (e.g., a circle or a parabola), the cylinder is called a quadric cylinder. The commonest type of cylinder is the right circular cylinder, in which the directrix is a circle and the lines forming the cylinder are all perpendicular to the plane of the circle. The solid bounded by a cylindrical surface and two parallel planes intersecting the surface in closed curves is also called a cylinder. The perpendicular distance between the planes is the altitude of the cylinder. The volume of the cylinder is equal to the product of the altitude and the area of the base (the area enclosed by either closed curve).

Cylinder may refer to:

• Cylinder (geometry), a three-dimensional geometric surface
• Cylinder (algebra), the cartesian product of a set with its superset
• Cylinder (engine), the space within which a piston travels in an engine
• Cylinder (steam locomotive)
• Cylinder (computer), a unit of data division on a hard disk drive
• Hydraulic cylinder, a cylinder barrel with piston rod for pushing and pulling using pressurized fluid
• Pneumatic cylinder, a cylinder barrel with piston rod for pushing and pulling using compressed gases
• Gas cylinder, containers used to store compressed gases or compressed air
• Diving cylinder, a type of gas cylinder used in Scuba diving
• Graduated cylinder, glassware used for measurement of liquids in laboratories
• Phonograph cylinder, the dominant medium of audio storage from the 1870s to the 1910s and in limited use (such as in the dictaphone) through the mid 20th century
• Cylinder (firearms), the cylinder is the rotating device that contains the firing chambers of a revolver
• A type of lens and a form of refraction error
• The part of a lock that is profiled to accept a specific key