linchpin

Noun

Synonyms: anchor mainstay keystone backbone lynchpin

WordNet 3.0

Noun (2)

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”Synonyms: anchor mainstay keystone backbone lynchpin
pin inserted through an axletree to hold a wheel onSynonyms: lynchpin

Webster's Dictionary (GCIDE)

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. [1913 Webster]