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.
彼はチームの要石だ。
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.