What is a "departure" in the context of flight testing / maneuvers?
Departure means that the aircraft has departed from controlled flight. From FAA news:


  A Loss of Control (LOC) accident involves an unintended departure of an aircraft from controlled flight


In this video about flight testing F-16s, the test pilot says 'intentionally departed from controlled flight' and later 'the aircraft is prone to departure if mishandled' and 'the aircraft departed in yaw'.