keystone
key-stone
noun
A fundamental, crucial part on which other elements depend for success.
"Teamwork is the keystone of our project's success."
noun
A wedge-shaped stone at an arch's apex, locking others in place.
"The keystone held the ancient arch together."
Thesaurus
Synonyms: key, headstone, anchor, mainstay, linchpin, lynchpin, backbone