The only possible sources of disintegration blasts are a black dragon's breath or a god's wide-angle disintegration beam (which can also destroy worn armor even if you are reflective or disintegration-resistant). There are no spells or wands of disintegration.
Reflection will reflect a blast of disintegration, but not a wide angle disintegration beam. You can gain disintegration resistance by eating an adult black dragon corpse, by wearing black dragon scales or black dragon scale mail, or by polymorphing into any black dragon. This protects you from both sources of disintegration, and it protects your armor from a dragon's blast. (Reflective or black dragon armor is not destroyed by a god's wrath.)
Magic resistance does not save you from disintegration.
- worn shield, otherwise:
- worn body armor (and cloak if present), otherwise:
- you (or the monster), plus cloak and shirt if present, and in the case of a monster, each inventory item (with probabilities of 95% for mundane objects, 50% for most artifacts, and 0% for your quest artifact and unique items). An amulet of life saving will revive its disintegrated wearer (unless the wearer is a nonliving monster, which can only occur if a living monster wearing an amulet is polymorphed).
The blast of disintegration is a ray which bounces off of walls and destroys closed doors. The same ray can affect a creature multiple times if it bounces.
Other uses of the term
Certain items can also disintegrate. A spellbook has a chance of disintegrating when you fail to read it. An uncursed scroll of scare monster disintegrates when picked up if it already has been picked up once, and a cursed one always disintegrates when picked up. In neither case does the game refer to the destruction as "disintegration".
As of NetHack 3.6.0, you will "disintegrate into a pile of dust" if you try to sacrifice the Amulet of Yendor on the high altar in Moloch's Sanctum and survive Moloch's first attempt to kill you, even if you have disintegration resistance.