foundation: the lowest supporting part of a built thing, the base it rests on or rises from
This word names the bottom of a built thing, the part that everything above it stands on. It appears in two settings, but the idea is the same in both.
In the law of sacrifice it is a fixed feature of the altar of burnt offering: the base or ledge at its foot, against which the leftover blood of certain offerings was poured out. Nine of its appearances belong to this ritual, and they read almost identically, "at the base of the altar."
Everywhere else it is the footing of walls, cities, and even a human life. Prophets and poets reach for it most often when something is being torn down: to strip a city "to its foundation" is to level it completely, and to lay bare or break the foundation is to bring the whole structure down. Set against that, the same word can picture what stands firm, as when the righteous person is called "an everlasting foundation."
It is close in sense to other Hebrew words for a base, such as מוֹסָדָה, and it comes from the verb יָסַד, "to found" or "to lay a foundation."