What does "Constant wind condition during flight" mean? What is constant wind condition during flight? Is it zero wind velocity at an instant (say at level flight) or wind with constant speed, say 20 m/sec (at steady level flight or whatever it is) Which statement is correct?
It means constant, or unchanging wind magnitude and direction.  It could be still air or just a constant upper level wind.  That can happen on local flights but is rare on long cross country flights.