A piezometer is a device used to measure the pressure of groundwater or the hydraulic head in soil or rock. It typically consists of a tube that is open at the bottom and filled with water, allowing for the measurement of water levels that indicate pressure changes in the surrounding environment. Piezometers are commonly used in hydrology, civil engineering, and environmental studies to monitor groundwater levels and assess the behavior of aquifers. They can be either standpipe piezometers or vibrating wire piezometers, depending on the desired accuracy and application.