linch·pin /ˈlɪnʧˌpɪn/
  制輪楔,車轄,關鍵
  Linch·pin n.  A pin used to prevent the wheel of a vehicle from sliding off the axletree.
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  linchpin
       n 1: a central cohesive source of support and stability; "faith
            is his anchor"; "the keystone of campaign reform was the
            ban on soft money"; "he is the linchpin of this firm"
            [syn: anchor, mainstay, keystone, backbone, lynchpin]
       2: pin inserted through an axletree to hold a wheel on [syn: lynchpin]