1. opposite side; bottom; other side; side hidden from view; undersurface; reverse side
葉の裏に小さな虫がいる。There's a small bug on the underside of the leaf.
2. rear; back; behind
校舎裏で別れた後、すぐに家に帰って自室の掃除を始めた。After parting ways behind the school building, I immediately went home and started cleaning my room.
3. lining; inside
4. in the shadows; behind the scenes; offstage; behind (someone's) back
皆、実は裏で自分を高めようと必死だったのさ。Everyone was actually desperately trying to improve themselves behind the scenes.
5. more (to something than meets the eye); hidden side (e.g. of one's personality); unknown circumstances; different side
6. proof
7. opposite (of a prediction, common sense, etc.); contrary
8. inverse (of a hypothesis, etc.)
9. bottom (of an inning); last half (of an inning)