Pronunciation: (king'pin"), [key]
b. the pin at the center; the number five pin.
2. Informal.the person of chief importance in a corporation, movement, undertaking, etc.
3. Informal.the chief element of any system, plan, or the like.
4. a kingbolt.
5. either of the pins that are a part of the mechanism for turning the front wheels in some automotive steering systems.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.