Flight Pack or Airborne Pack : All of the radio equipment installed in the airplane, i.e., Receiver, Servos, Battery, Switch Harness.
This is not a similar thing to the flight box, rather it refers to all the radio equipment that is used in an RC aircraft during flight. This includes most importantly the receiver, but also the servos/ESC, flight pack battery/BEC and the harness. The transmitter is outwith the craft and so not considered part of the flight pack, despite its involvement.
Flight Pack Airborne Pack