A shade is an undead creature, created by black magic.
It cannot be harmed except with blessed or silver objects, magic, artifacts or a mirror (which does 1 point of damage and breaks; according to the source there is "silver in the reflective surface"). Orcus-town is the only place where shades appear.
If you've gotten this far without an alternative, you can use the Bell of Opening or any other silver item that isn't a silver arrow (unless you're shooting the arrows with your bow) against them. Unarmed gloveless combat with a silver ring equipped can also work, as can unarmed combat with blessed gloves (although this is very slow), or simply wielding the silver ring (regardless of whether or not you are wearing gloves). Of these alternatives, wielding a silver object is most advisable, because it gives you d20 silver damage as opposed to only d4 blessed damage and does not require you to remove your armor. A non-weapon silver object will not break wielded weaponless conduct.
Shades are undead creatures. They differ from zombies in
that a zombie is an undead animation of a corpse, while a
shade is an undead creature magically created by the use
of black magic.