The ability of a material or structure to absorb heat energy is called Thermal Mass.
High density materials such as stone, concrete, bricks, and tiles require a great deal of heat energy to change their temperature and are therefore said to have high thermal mass.
Thermal mass acts as in the same way as a battery, except that is stores heat instead of electricity.
Thermal mass stores and re-radiates heat.
from David Darling’s Encyclopedia of Alternative Energy
