"Success is a lousy teacher. It seduces smart people into thinking they can't lose." ~ Bill Gates (American Entrepreneur and Founder of Microsoft Co., b.1955)

linchpin (or lynchpin)
Pronunciation: 'linch-"pin
Function: noun
Etymology: Middle English lynspin, from lyns linchpin (from Old English lynis) + pin; akin to Middle High German luns linchpin
1 : a locking pin inserted crosswise (as through the end of an axle or shaft)
2 : one that serves to hold together the elements of a complex