Underpinning

Definition:

(p. pr. & vb. n.) of Underpin
(n.) The act of one who underpins; the act of supporting by stones, masonry, or the like.
(n.) That by which a building is underpinned; the material and construction used for support, introduced beneath a wall already constructed.
(n.) The foundation, esp. of a frame house.

Compare underpinning with other words:

underlying vs. underpinning