A gas cylinder is a pressure vessel for storage and containment of gases at above atmospheric pressure. High-pressure gas cylinders are also called bottles. Inside the cylinder the stored contents may be in a state of compressed gas, vapor over liquid, supercritical fluid, or dissolved in a substrate material, depending on the physical characteristics of the contents. A typical gas cylinder design is elongated, standing upright on a flattened bottom end, with the valve and fitting at the top for connecting to the receiving apparatus.

The term cylinder in this context is not to be confused with tank, the latter being an open-top or vented container that stores liquids under gravity, though the term scuba tank is commonly used to refer to a cylinder used for breathing gas supply to an underwater breathing apparatus.