A thermistor is a temperature-sensitive resistor. Its resistance changes when temperature changes, which lets a circuit estimate or respond to temperature.
NTC and PTC thermistors
An NTC thermistor has resistance that falls as temperature rises. A PTC thermistor has resistance that rises as temperature rises. Each type is useful in different circuit designs.
Where thermistors are used
Thermistors appear in thermostats, battery packs, chargers, 3D printers, appliances, medical devices, and temperature monitoring circuits. A microcontroller can read a thermistor through a voltage divider.
A thermistor is a simple way for electronics to sense heat.