As used in this chapter:
(a) “Witness” includes any person whose testimony is desired in any proceeding or investigation by a grand jury or in any criminal action, prosecution, or proceeding.
(b) “State” means any State or Territory of the United States and the District of Columbia.
(c) “Grand jury investigation” means any grand jury investigation which has commenced or is about to commence.
(d) “Per diem” means a sum of money the purpose of which is to provide for personal expenses, including, but not limited to, food and lodging.