A thermistor is a type of temperature sensor commonly used in hotends for 3D printers. It is a small device that measures the temperature of the hotend and sends this information back to the printer's control board. This allows the printer to maintain a consistent and accurate temperature for the hotend, which is important for producing high-quality prints. The thermistor is typically attached to the hotend using a piece of insulating material, such as a silicone sleeve or heat-resistant tape. Some hotends may have the thermistor built-in, while others require it to be installed separately.