When a regenerate spell is cast, body members (fingers, toes, hands, feet, arms, legs, tails, or even heads of multi-headed creatures), bones, and organs grow back. The process of regeneration requires but a few minutes if the severed member(s) is (are) present and touching the creature, an hour or two otherwise. The creature must be living to receive the benefits of this spell.
The reverse, wither, causes the member or organ touched to cease functioning, eventually dropping off into dust.
Note: This spell originally appeared in this form in the 2nd Edition Player's Handbook for Advanced Dungeons & Dragons from TSR. Its use here is for the purposes of providing context for the campaign only.