Apply, a, is a command to 'use' an item. For example, you can apply a unicorn horn to remedy confusion, blindness, sickness, etc. You can also apply a pick-axe to break down a wall, a lamp to light it, a magic marker to write, etc.

Wands are used by zapping them, z. Applying a wand breaks it, causing an explosion that damages you and nearby monsters. You need to have a minimum strength of 10 to break a wand.

Most weapons are used by throwing, firing, or by first wielding and then attempting to walk into an opponent. The exception is polearms, which must be wielded, then applied from exactly two squares away. See the article on pounding for details.

Artifacts may have powers that can be accessed by the apply command, and may have additional powers that can be accessed by the invoke command. See, for example the Orb of Fate, which acts as a crystal ball when applied and provides level teleportation when invoked.