The following words and phrases when used in this subchapter shall have the meanings given to them in this section unless the context clearly indicates otherwise:
"Attack." Any attack on the United States which causes or may cause substantial damage or injury to civilian persons or property in any manner by sabotage or by the use of bombs, missiles or shellfire or by atomic, radiological, chemical, bacteriological or biological means or other weapons or processes.
"Emergency interim successor." A person designated, in the event the officer is unavailable, to exercise the powers and discharge the duties of an office until a successor is appointed or elected and qualified as may be provided by the Constitution of Pennsylvania, statutes, charters and ordinances or until the lawful incumbent is able to exercise the powers and discharge the duties of the office.
"Office." All local offices, the powers and duties of which are defined by the Constitution of Pennsylvania or a statute, charter or ordinance.
"Unavailable." When a vacancy in office exists and there is no deputy authorized to exercise all of the powers and discharge the duties of the office or when the lawful incumbent of the office or any deputy exercising the powers and discharging the duties of an office because of a vacancy and his authorized deputy are absent or unable to exercise the powers and discharge the duties of the office.