The instrument used to measure atmospheric pressure is called a "barometer." Barometers are essential tools in meteorology, helping to predict weather changes by measuring the weight of the atmosphere above a given point. There are different types of barometers, including mercury barometers, which use a column of mercury to measure air pressure, and aneroid barometers, which use a small, flexible metal box that expands or contracts with changes in pressure. Understanding atmospheric pressure is crucial for weather forecasting, aviation, and various scientific research applications.