存水弯，是一个U型、S型、或J形管下方或在管件设备，让废水及污水流进地下室筏基，并且阻止臭气及其他气体逆流或倒灌进室内， Alexander Cummings在1775年发明S型弯的，但汤马斯·克拉普1880年引进U形存水弯。

由于它的形状，存水弯使用之后，保留了少量的水。存水弯形成密封，防止下水道气体通过排水管进入建筑空间。基本上所有的管道设备，包括水槽，浴缸，厕所必须配备内部或外部的存水弯。

存水弯也往往捕捉重物（如珠宝）不慎掉进水槽。存水弯也倾向于收集头发，沙子等杂物，并限制传递到其余的管道，大多数存水弯可以被拆卸进行清洗或它们提供了某种形式的清洗功能。

In plumbing, a trap is a U-, S-, or J-shaped pipe located below or within a plumbing fixture. An S-shaped trap is also known as the S-bend invented by Alexander Cummings in 1775 but became known as the U-bend following the introduction of the U-shaped trap by Thomas Crapper in 1880. The new U-bend could not jam, so, unlike the S-bend, it did not need an overflow. The bend is used to prevent sewer gases from entering buildings. In refinery applications, it also prevents hydrocarbons and other dangerous gases from escaping outside through drains.