linchpin

linchpin \LINCH-pin\, noun:

1. something that holds the various elements of a complicated structure together: The monarchy was the linchpin of the nation's traditions and society.
2. a pin inserted through the end of an axletree to keep the wheel on.

Sometimes I feel if I lost it I would lose the linchpin of my life. But of course I remember that in one way I lost the linchpin years before, not long after I acquired the datebook. It was not an even swap.

-- Anna Quindlen, One True Thing, 2010

… let's not forget who started the whole thing, who was the first one to go out of his element and drown, whose watery death removed the linchpin, the foundation-stone, and began the family's long slide, which ended up by dumping me in the pit: Francisco de Gama, Epifania's defunct spouse.

-- Salman Rushdie, The Moor's Last Sigh, 1995

Linchpin, a portmanteau of lynch and pin, comes from the Old English lynis. While the literal sense entered English in the late fourteenth century, the figurative sense has supplanted the original sense in most contexts since the 1950s.