linchpin

Literally, a linchpin is the pin that goes through the axle of a wheel to keep it in place, but linchpin can be used to mean an important part of anything, the thing that holds it all together.

The linchpin is something that supports and holds another thing together. It’s the most important part of anything — eating well is the linchpin to staying healthy. A basketball team's most valuable player is the linchpin. The most crucial part of a plan is the linchpin. If something or someone is indispensable, she’s the linchpin. Take away the linchpin and everything falls apart.

Definitions of linchpin
1

n a central cohesive source of support and stability

“he is the linchpin of this firm”
Synonyms:
anchor, backbone, keystone, lynchpin, mainstay
Type of:
support
something providing immaterial assistance to a person or cause or interest

n pin inserted through an axletree to hold a wheel on

Synonyms:
lynchpin
Type of:
pin
a small slender (often pointed) piece of wood or metal used to support or fasten or attach things

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