/linch"pin'/ , n.
1. a pin inserted through the end of an axletree to keep the wheel on.
2. something that holds the various elements of a complicated structure together: The monarchy was the linchpin of the nation's traditions and society.
Also, lynchpin .
[ 1350-1400; unexplained alter. of ME lynspin, equiv. to lyns, OE lynis axle-pin (c. G Lünse ) + pin PIN ]