In the development of the second law of thermodynamics, it’s convenient to have a hypothetical body with a relatively large thermal energy capacity
that can supply or absorb finite amounts of heat without undergoing any change in temperature. Such a body is called a thermal energy reservoir.
Large bodies of water (oceans, lakes, rivers) and atmospheric air can be modeled accurately as thermal energy reservoirs; for example, the atmosphere doesn’t warm up as a result of heat losses from residential buildings in the winter.
A reservoir that supplies energy in the form of heat is called a source, and one that absorbs energy is a sink.