中堅
ちゅうけん nucleus; backbone
Why it means this
中 (the middle) + 堅 (solid, firm) — from the well-guarded centre of an old army formation; the solid core of an organization — mid-career staff, mid-sized firms; also centre field in baseball (中堅企業; 中堅社員).
Breakdown
Each character links to its full entry — origin story, stroke order, and readings.