压力容器通常是指一个专门设计能承受一定压力载荷，以盛装气体或液体的密闭容器，材质包括金属及非金属材料。压力容器内部和外部的压力差具有潜在的危险。在压力容器的发展历史上，许多安全事故都因为设计、制造、操作和使用不当而发生。因此压力容器受到严格的标准控制。由于这些原因，压力容器的定义因为每个国家的标准及所涉及的参数不同而有差异，例如最大安全工作压力和温度。

中华人民共和国国家质量技术监督局1999年制定的《压力容器安全技术监察规程》(已有新版)规定：

A pressure vessel is a 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. Consequently, pressure vessel design, manufacture, and operation are regulated by engineering authorities backed by legislation. For these reasons, the definition of a pressure vessel varies from country to country, but involves parameters such as maximum safe operating pressure and temperature, and are engineered with a safety factor, corrosion allowance, minimum design temperature (for brittle fracture), and involve nondestructive testing, such as ultrasonic testing, radiography, and pressure tests, usually involving water, also known as a hydrotest, but could be pneumatically tested involving air or another gas. The preferred test is hydrostatic testing because it's a much safer method of testing as it releases much less energy if fracture were to occur (water does not rapidly increase its volume while rapid depressurization occurs, unlike gases like air, i.e. gasses fail explosively). In the United States, as with many other countries, it is the law that vessels over a certain size and pressure (15 PSIg) be built to Code, in the United States that Code is the ASME Boiler and Pressure Vessel Code (BPVC), these vessels also require an Authorized Inspector to sign off on every new vessel constructed and each vessel has a nameplate with pertinent information about the vessel such as maximum allowable working pressure, maximum temperature, minimum design metal temperature, what company manufactured it, the date, it's registration number (through the National Board), and ASME's official stamp for pressure vessels (U-stamp), making the vessel traceable and officially an ASME Code vessel.