A Meteorite is an item that can change a Deoxys's form when used on the Deoxys. It can only be used on a Deoxys owned by the player using the Meteorite. The Meteorite can be used an infinite number of times to change a Deoxys's form.
A Meteorite is held by every wild Deoxys, thus one is obtained when any Deoxys is captured.
The Meteorite can be crafted with the items in the image below