Heater is used to increase the temperatures of something e.g water. Now on the thermometer it is used to measure the temperatures e.g Celsius of a person or something for example the water.
In simple terms, a heater is an object that heats up. A thermometer, on the other hand, measures heat.

Thermometers are designed to measure specific temperatures, such as the temperature of your body or the temperature of water. Thermometers can also be used to measure humidity.

Heaters are designed to raise the temperature of a specific area. Heater units are typically installed in homes and offices to keep people warm in cold weather or cool in hot weather.