org-deadline-string

org-deadline-string is a variable defined in `org.el'.
Its value is

"DEADLINE:"

Documentation:
String to mark deadline entries.
A deadline is this string, followed by a time stamp. Should be a word,
terminated by a colon. You can insert a schedule keyword and
a timestamp with M-x org-deadline.
Changes become only effective after restarting Emacs.

You can customize this variable.