PIC
Pilot in Command
The pilot legally responsible for the operation and safety of the aircraft during flight.
Pilot in Command is defined in FAR 1.1 as the person who has final authority and responsibility for the operation and safety of the flight, has been designated as PIC, and holds the appropriate certificate, rating, and currency for the conduct of the flight.
PIC is also a flight time category for logbook purposes. Under FAR 61.51, you may log PIC time when you are the sole manipulator of the controls of an aircraft for which you are rated, when you are the sole occupant of the aircraft, or when you are acting as PIC of an aircraft requiring more than one pilot.
The two uses sometimes overlap and sometimes don't. A student pilot can log PIC for solo time but isn't "acting as PIC" in the regulatory sense the same way a certificated pilot is.
When it matters
PIC currency is tracked separately from total time for some requirements. Flying clubs and schools track PIC hours per member to determine instructor and aircraft access.