point d'appui [Fr. pwaN' dapwi] — n.
1.) A foundation, a base.
2.) The base or rallying point for a military unit.
Etymology — Modern French: literally, point of support.
"Let us settle ourselves, and work and wedge our feet downward through the mud and slush of opinion, and prejudice, and tradition, and delusion, and appearance, that alluvion which covers the globe, through Paris and London, through New York and Boston and Concord, through Church and State, through poetry and philosophy and religion, till we come to a hard bottom and rocks in place, which we can call reality, and say, This is, and no mistake; and then begin, having a point d'appui, below freshet and frost and fire, a place where you might found a wall or a state, or set a lamp-post safely, or perhaps a gauge, not a Nilometer, but a Realometer, that future ages might know how deep a freshet of shams and appearances had gathered from time to time" (Walden, Henry David Thoreau, 1854).