Definition
The pilot-in-command is the person who has final authority and responsibility for the operation and safety of a flight, holds the appropriate category, class, and type ratings (when required) for the aircraft, and is designated as PIC before or during the flight. Under 14 CFR §91.3, the PIC is directly responsible for, and is the final authority as to, the operation of the aircraft.
Plain English
The pilot who is in charge of the flight. Whatever happens, the buck stops with this person — they make the final calls and carry the responsibility for the aircraft and everyone on board.
Context Anchor
You will see PIC in flight training, aircraft checkouts, logbook entries, regulations, and discussions about who is responsible for a flight.
Derivation
From 'pilot' (the person flying) and 'in command' (in charge). The phrase 'in command' comes from military and maritime tradition, where the commander of a vessel holds final authority regardless of who else is aboard. Aviation borrowed this directly: only one person can be in command of an aircraft at any moment.
Why Pilots Care
The PIC holds legal responsibility for all flight decisions and safety outcomes.
Intuition Check
Do not assume the PIC is always the person holding the controls. The PIC is the pilot with final responsibility for the flight, even if someone else is flying at that moment.
Example Sentence 1
Before engine start, the two pilots agreed that the captain would act as pilot-in-command for the outbound leg.
Example Sentence 2
Although the student was flying the airplane, the instructor remained the PIC.