Persons employed by the air operator primarily for the purpose of exercising the privileges of an FAA pilot certificate issued under FAR 61 and assigned primary responsibilities for operation and safety of an aircraft during flight.  These persons will be qualified and trained as a PIC (Pilot in Command) by the air operator.
Here is the FAA’s official definition of PIC:
1. The PIC has the final authority and responsibility for the operation and safety of the flight;
2. Has been designated as pilot in command before  or during the flight; and
3. Holds the appropriate category, class, and type rating, if appropriate, for the conduct of the flight.