keystone; linchpin; cornerstone
Refers to the central stone at the top of an arch that locks the others in place; also used figuratively for the most important part of something.
Without the keystone of the arch, the whole thing would collapse.
He is the linchpin of the team.
Compound of 要 (kaname, 'pivot, essential point') and 石 (ishi, 'stone'). The exact historical derivation is uncertain, but the word directly describes the keystone in an arch.