1. nucleus; backbone; mainstay; key figure
2. medium-level; mid-level; middle-ranking; midsize
3. main body (of troops); crack troops; select troops
4. center field; centre field; center fielder; centre fielder
5. athlete competing in the middle-number match in a team competition, i.e. second in 3-on-3, third in 5-on-5 (kendo, judo, etc.)