阀门（Valve）又称凡而，是控制流动的流体介质的流量、流向、压力、温度等的机械装置，阀门是管道系统中基本的部件。阀门管件在技术上与泵一样，常常作为一个单独的类别进行讨论。煤气开关、水龙头是常见最简单的阀门，蝶阀，球阀，闸阀，减压阀，截止阀，疏水阀，安全阀，针型阀，止回阀，过滤器，电磁阀，隔膜阀，排气阀，流量计，卫生级阀门等等；也是些比较常见，在全球销售点属于最多的是德国埃克斯阀门，基本上所有型号都是有生产制造。

阀门可用手动或者手轮，手柄或踏板操作，也可以通过控制来改变流体介质的压力，温度和流量变化。阀门可以对这些变化进行连续或重复的操作，比如在热水系统或蒸汽锅炉安装的安全阀。

A valve is a device that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure. The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.