1. medium (size); average (grade, level, etc.); middle
サイズは大、中、小の三つです。There are three sizes: large, medium and small.
2. in; within; out of (e.g. ten people); among
3. during; while; in (the course of); within (a period); throughout
数日中にこちらに届くはずです。It should arrive here within a few days.
4. currently (doing); in the process of; in the middle of; under (e.g. construction, investigation)
とはいえ、まだまだ成長中だからな。That said, I'm still growing.
5. middle way; moderation
6. second volume (of three)
7. junior high school; middle school
8. China