Millimeters, water gauge
Millimeters, water gauge, also known as millimetres water or millimetres water column and abbreviated to mm wg, mm H2O or mm wc respectively, is a unit of pressure. It is the pressure required to support a water column of the specified height (sometimes called 'head' of water). It is equivalent to 9.80665 Pa. In limited and largely historic contexts it may vary with temperature, using the equation:
The unit is often used to describe how much water rainwear or other outerwear can take or how much water a tent can resist without leaking.
...
Wikipedia