Thermal mass in the most general term refers to any mass used to absorb and hold heat. In architectural terms, it is a passive solar principle.
Thermal mass is used in and around buildings to absorb or emit heat. Internal thermal mass in a well-insulated building, such as concrete or other forms of masonry, or water, reduces temperature swings in the interior. When thermal mass is placed where sunlight shines directly on it during winter months, it absorbs the sun's energy and re-radiates it as heat energy.
Earth sheltering is another important architectural means of using thermal mass. By placing exterior walls in direct juxtaposition to earth, the thermal mass of the earth provides a fairly constant, moderating temperature so that heat flow through the adjacent wall is greatly slowed.