A thermometer is an instrument that measures temperature, indicating how hot or cold something is, and works by using a substance (like liquid or a thermistor) that expands or changes electronically with heat, displayed on a numerical scale or digital screen. Common types include liquid-in-glass (mercury/alcohol), digital (using sensors), and infrared (no-contact), used in homes, labs, and hospitals for everything from cooking to monitoring fever, with principles based on thermal expansion or electronic sensing.