The instrument that measures temperature is called a "thermometer." Thermometers can operate on various principles, with the most commonly known types being mercury and digital thermometers. Mercury thermometers use the expansion of mercury in a glass tube to measure temperature, while digital thermometers use electronic sensors to provide a quick reading. Temperature measurement is essential in various fields, including meteorology, medicine, and cooking. Accurate temperature readings help ensure the safety and quality of food, monitor patient health, and contribute to scientific research, making thermometers vital tools in day-to-day life.