Layer of the atmosphere above 85 km.
The thermosphere can be considered the top layer of the Earth's atmosphere. The layer above the thermosphere is called the exosphere and may or may not be considered part of the atmosphere.
Since the thermosphere is very high altitudes and is a near vacuum, it gives a good demonstration of the difference between temperature and heat. Since the layer is so high it gets a lot of solar radiation and individual molecules are very high energy. The temperature of these molecules can reach almost 5000 degrees F. This is where the layer gets its name. However, an astronaut requires thermal protection to prevent freezing to death. This is because there are so few molecules and they are so far apart that the high energy doesn't amount to much.
For example, take a regular carpentry nail and heat it up to 2000 F, then drop it in a 5 gallon pail of water. The red hot nail will barely raise the temperature of the water more a degree or 2. If you take a 10 lb iron weight and heat it up to 500 F and then drop it into the same pail the temperature of that water will rise much much higher than from the same nail.