Mass. Gen. Laws ch. 263A § 1
For the purposes of this chapter, the following words shall have the following meanings:-
"Board", the witness protection board established in section 2.
"Prosecuting officer", the attorney general or a district attorney from each county.
"Critical witness", any person who is participating, has participated, or is reasonably expected to participate in a criminal investigation, motion hearing, trial, show cause hearing, or other criminal proceeding, or a proceeding involving an alleged violation of conditions of probation or parole, or the commitment of a sexually dangerous person pursuant to chapter 123A; or who has received a subpoena requiring such participation; who is, or was, in the judgment of the prosecuting officer, a necessary witness at one or more of the aforementioned types of proceedings, and who is or may be endangered by such person's participation in the aforementioned proceeding; or such person's relatives, guardians, friends or associates, who are or may be endangered by such person's participation in the aforementioned proceeding.
Mass. Gen. Laws ch. 263A, § 1