1. to get off (Bikes, buggies or jet-skies. NOT for cars, busses and trains. That's "aussteigen". It works for horses, but "absitzen" is the more "jargon" term there.)
Opposite (closest): aufsteigen
2. to descend (In context of mountain climbing, but very rare. The noun "der Abstieg" is more common.)
3. to be relegated (In context of sports leagues, when a team goes one league "lower" at the end of a season.)
Opposite (closest): aufsteigen