All engineered construction resting on the earth must be carried by some kind of interfacing element called foundation. The foundation is the part of an engineered system that transmits to, and into the underlying soil or rock the load supported by foundation and its self weight, the resulting soil stresses - except at the ground surface - in addition to those presently exist in the earth mass from its self weight and geological history.
The term super structural is commonly used to describe the engineered part of the system bringing load to the foundation, or sub structure. It is better To describe the foundation as the part of engineered system that interfaces the load -carrying components to the ground.
Foundation have the function of spreading the load from the superstructure so that the pressure transmitted to the ground is not of a magnitude such as to cause the ground to fail in shear, or to induce settlement of the ground that will cause distortion and structural failure or an acceptable architectural damage. In fulfilling these functions the foundation, substructure and superstructure should be considered as one unit. The tolerable total and differential settlement must be related to the type and use of structure and its relationship to the surroundings.