noun
also lynch·pin ˈlinchˌpin
Etymology: Middle English lynspin, from lyns, lins linchpin (from Old English lynis ) + pin; akin to Old Saxon lunisa linchpin, Old High German lun, Sanskrit āṇi — more at ell
1.
a. : a pin inserted in an axletree outside of the wheel to prevent the latter from slipping off
b. : a linking device (as for making a plow rig fast to a tractor) that consists essentially of a metal pin that can be fitted into sockets in the objects to be linked and usually has some locking device to prevent shifting
2. : something that serves to hold together the elements of a situation
the linchpin in the prosecution's case was a subpoenaed canceled check — Joel Sayre
the linchpin in this entire policy is clearly collaboration — Joseph Barnes