What instrument is referred to by the term manometer?

A manometer is an instrument specifically designed to measure pressure, particularly the pressure of gases or liquids. It usually takes the form of a U-shaped tube filled with a fluid, such as mercury or water, where the difference in height of the fluid in the two arms of the tube reflects the pressure difference. This ability to gauge pressure is crucial in various applications, including medical settings for monitoring blood pressure and in laboratories to measure the pressure of gases in chemical experiments.

The other choices pertain to different measurements: temperature, weight, and volume are measured using thermometers, scales, and graduated cylinders or volumetric flasks, respectively. Thus, the defining characteristic of the manometer lies in its function to quantify pressure, which makes it the correct answer.