Linchpin

['lɪn(t)ʃpɪn] or ['lɪntʃpɪn]

(noun.) pin inserted through an axletree to hold a wheel on.

Inputed by Clinton--From WordNet

(n.) A pin used to prevent the wheel of a vehicle from sliding off the axletree.

Checked by Evita

n. a pin used to keep the wheel of a carriage on the axle-tree.

Editor: Martin