Coherence ?
I've seen one coherent definition (in which ground speed = ground speed), and something else which is hard to describe. Some even believe that it is necessary to hold the nose up a bit to get the necessary lift for level flight. Given that the earth is not flat, it is clearly necessary in fact to maintain a continuous descent to maintain a constant height above ground, with a bit of a bank to the left to counter coriolis when flying north in the northern hemisphere. That's why INS systems are so expensive as they take this drudgery away.