The International Civil Aviation Organization (ICAO) definition is: "The pilot responsible for the operation and safety of the aircraft during flight time.
2.3.1, specifically empowers the PIC to override any other regulation in an emergency, and to take the safest course of action at his/her sole discretion.
This provision mirrors the authority given to the captains of ships at sea, with similar justifications.
It essentially gives the PIC the final authority in any situation involving the safety of a flight, irrespective of any other law or regulation.
[3]: 717 Time logged as "pilot-in-command under supervision" (PICUS) may be partly used towards the hour requirements for the issue of an airline transport pilot licence (ATPL).