axis; pivot; central figure; key person
Used both literally for a mechanical axis/pivot and figuratively for a person or thing that serves as the central support of an organization or activity.
彼はチームの中軸として活躍している。
He is active as the central figure of the team.
The axis of this machine has become bent.
He is the player who serves as the team's central pivot.
The company built its business around the new technology as its central pivot.
She is bearing the central role in the project.