5 U.S.C. § 2104

(a) For the purpose of this title, “officer”, except as otherwise provided by this section or when specifically modified, means a justice or judge of the United States and an individual who is— (1) required by law to be appointed in the civil service by one of the following acting in an official capacity— (A) the President; (B) a court of the United States; (C) the head of an Executive agency; or (D) the Secretary of a military department; (2) engaged in the performance of a Federal function under authority of law or an Executive act; and (3) subject to the supervision of an authority named by paragraph (1) of this section, or the Judicial Conference of the United States, while engaged in the performance of the duties of his office. (b) Except as otherwise provided by law, an officer of the United States Postal Service or of the Postal Regulatory Commission is deemed not an officer for purposes of this title.