Foundation: The lowest artificially build part of a structure which transmits the load of the structure to the soil lying underneath is called foundation.
The structure such as walls, piers* are subjected to the loads of roof, floor or bridge etc. The load which is acting on a wall or a pier is to be transferred to the soil lying underneath. The load thus acting on the soil is to be distributed over a large area so that the intensity of loading comes within the ultimate bearing capacity of the soil. For this purpose a structure is provided with artificial arrangement in the form of concrete bed or block etc. at the base below ground level which act as foundation of structure.
The solid ground on which the foundation rests is called the foundation bed.
Ultimate bearing capacity of the soil :
The maximum load which the soil can take per unit area without yielding or displacement is called bearing capacity or ultimate bearing capacity of soil.
It is usually measures in terms of tonne per square meter.
pier* A pillar supporting an arch or bridge.
Reference: Building construction(in SI units) by N.L.Arora and B.R.Gupta