These definitions and illustrations are from the passive solar page of Malcolm Wells' website. These two particular concepts are essential to designing an energy-efficient house.
Thermal mass Thermal mass
Materials that store heat. Heavy dense materials—concrete, stone, and even water—store a lot of heat in a small volume (compared with most lightweight materials) and release it when needed.
Mean Radiant Temperature
The average temperature you experience from the combination of all the various surface temperatures in a room: walls, floors,
Mean Radiant Temp.
If your inside air temperature is comfortable but the walls, furniture, floors, etc. are cold, then you will feel cold. Thermal mass keeps temperatures steady. If a cold north wind comes up suddenly, you won't rush to the thermostat if the shell of your home is insulated concrete.
Carpenter's house has been a constant 71 degrees this last week, which feels fine because the walls, floor, ceiling and the air are all in sync. It doesn't matter whether it's 40 degrees outside with a howling north wind or whether it's 60 degrees, calm and sunny. The inside maintains its temperature and stays comfortable.