The following words and phrases when used in this subchapter shall have, unless the context clearly indicates otherwise, the meanings given to them in this section:
"Penal institution." Includes a jail, prison, penitentiary, house of correction, correctional institution or other place of penal detention.
"Witness." A person who is confined in a penal institution in any state and whose testimony is desired in another state in any criminal proceeding or investigation by a grand jury or in any criminal proceeding before a court.