Definition: linchpin

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Source: WordNet (r) 1.7 

linchpin
     n 1: a central cohesive source of support and stability: "faith
          is his anchor"; "he is the linchpin of this firm" [syn:
          anchor, mainstay, backbone, lynchpin]
     2: pin inserted through an axletree to hold a wheel on [syn: lynchpin]

Source: Webster's Revised Unabridged Dictionary (1913) 

Linchpin \Linch"pin`\ (l[i^]nch"p[i^]n`), n. [AS. lynis the
   axletree; akin to D. luns linchpin, OS. lunisa, LG. lunse, G.
   l["u]nse, OHG. lun peg, bolt.]
   A pin used to prevent the wheel of a vehicle from sliding off
   the axletree.