Footing
The widened concrete base beneath a wall or column that spreads the structure's load over enough soil area to prevent settlement.
What does footing mean?
Footing size follows from the load above and the bearing capacity of the soil below, which is why the same house needs a larger footing on soft clay than on dense gravel. Footings must bear on undisturbed soil or properly compacted fill, and they must sit below the local frost depth in freezing climates. Residential codes provide prescriptive tables that cover ordinary conditions without engineering.
In practice
The inspector who walks a footing trench before the pour is checking depth, width, soil condition, and reinforcement, in roughly that order.