1.
mid-level; mid-career; core member
A person or group that forms the main body of an organization, between newcomers and senior management.
中堅社員として活躍している。
I'm active as a mid-level employee.
中堅企業で働いている。
I work at a mid-sized company.
彼はチームの中堅として重要な役割を果たす。
He plays an important role as a core member of the team.
2.
center fielder
In baseball, the player who covers center field.
FORMATION:
中 (middle) + 堅 (firm, solid)
USAGE:
- 中堅社員 (mid-career employee)
- 中堅企業 (mid-sized company)
- 中堅{どころ} (the backbone, core group)
- 中堅層 (middle tier)
NUANCE:
Implies reliability and experience. Often refers to employees with 5-15 years of experience who are neither new nor in senior management.
BASEBALL:
中堅手 - center fielder (formal)
センター - center fielder (common)