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:
"Summons." Includes a subpoena, order or other notice requiring the appearance of a witness.
"Witness." Includes a person whose testimony is desired in any proceeding or investigation by a grand jury or in a criminal action, prosecution or proceeding.
42 Pa.C.S. § 5962