Rampant (or Rampancy) refers to a state of the zanpakutō where a zanpakutō spirit loses its connection with its wielder and goes insane (thus "rampant"). When rampant, a spirit can potentially become far more dangerous than its original wielder, since no limits on its development of power exist for such a state, which caused the Central 46 Chambers to decree that any rampant spirits found must be destroyed. Tenkuko is the only zanpakutō spirit with the inherent ability to resist the insanity that comes from being disconnected with one's wielder, only due to the nature of her powers.
To reverse the effects of the rampancy, the wielder of the zanpakutō has to be able to approach the spirit and transfer a certain amount of one's own soul into the spirit. Theoretically, it should cause the spirit to remember its past and return to its sealed state, though there are few, if any, recorded accounts of the feat being done.