Is it ensure, insure, or assure?
fulcrum ful·crum (ful'krəm, fŭl'-)
n. pl. ful·crums or ful·cra (-krə)
The point or support on which a lever pivots.
An anatomical structure that acts as a hinge or a point of support.
The point on which a lever is balanced when a force is exerted.