What's The Pressure Vessel?

1.  pressure vessel is a container designed to hold material at high pressures or  an enclosed structure containing a nuclear reactor core immersed in pressurized coolant.

A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. The pressure differential is dangerous, and fatal accidents have occurred in the history of pressure vessel development and operation.

en.wikipedia.org/wiki/Pressure_vessel

A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.

A pressure vessel is defined as"a vessel in which the pressure is obtained from an indirect source or by the application of heat from an indirect source or a direct source. The vessel proper terminates at: (a) the first circumferential joint for welded end connections; (b) the face of the first flange in bolted flange connections; or (c) the first threaded joint in threaded connections."

Pressure vessels include but are not limited to compressed gas storage tanks (i.e., air, oxygen, nitrogen tanks, etc.), anhydrous ammonia tanks, hydro pneumatic tanks, autoclaves, hot water storage tanks, chemical reactors and refrigerant vessels, designed for a pressure greater than15 psi and a volume greater than5 cubic feet in volume or one and one-half cubic feet in volume with a pressure greater than600 psi.

Agreed with Mr. Alex Answer  "a vessel in which the pressure is obtained from an indirect source or by the application of heat from an indirect source or a direct source.