linchpin

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The Word of the Day for November 20 is: 

linchpin   \LINCH-pin\   noun
     1 : a locking pin inserted crosswise (as through the end of an axle or 
shaft)
    *2 : one that serves to hold together parts or elements that exist or 
function as a unit

Example sentence:
     Jane's canceled check was the linchpin of the case against her, because it 
proved that she did know about the sale of Aunt Jessie's vase.

Did you know?
     "There was the good old custom of taking the linch-pins out of the 
farmers' and bagmens' gigs at the fairs, and a cowardly blackguard custom it 
was." That custom, described by British writer Thomas Hughes in his 1857 novel 
_Tom Brown's School Days_, was "blackguard" indeed. The linchpin in question 
held the wheel on the carriage and removing it made it likely that the wheel 
would come off as the vehicle moved. Such a pin was called a "lynis" in Old 
English; Middle English speakers added "pin" to form "lynspin." Modern English 
speakers modified it to "linchpin" and, in the mid-20th century, began using 
the term figuratively for anything as critical to a complex situation as a 
linchpin is to a wagon.

*Indicates the sense illustrated in the example sentence.