org-archive-tag

org-archive-tag is a variable defined in `org.el'.
Its value is

"ARCHIVE"

Documentation:
The tag that marks a subtree as archived.
An archived subtree does not open during visibility cycling, and does
not contribute to the agenda listings.
After changing this, font-lock must be restarted in the relevant buffers to
get the proper fontification.

You can customize this variable.