Wither is a keyword ability.
- 702.79a Wither is a static ability. Damage dealt to a creature by a source with wither isn’t marked on that creature. Rather, it causes that many -1/-1 counters to be put on that creature. See rule 119.3.
- 702.79b If a permanent leaves the battlefield before an effect causes it to deal damage, its last known information is used to determine whether it had wither.
- 702.79c The wither rules function no matter what zone an object with wither deals damage from.
- 702.79d Multiple instances of wither on the same object are redundant.