org-scheduled-string

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

"SCHEDULED:"

Documentation:
String to mark scheduled TODO entries.
A schedule 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-schedule.
Changes become only effective after restarting Emacs.

You can customize this variable.