Delta is simply the difference. I leave point A, arrive at point B some time later. The delta in terms of time is the difference between the time of departure from A and arrival at B.
It can also be any other measure. The Delta between A and B could be the mileage between the 2 points. Or even the difference in elevation. Simply put, the difference.
It is used fairly often by Q and those commenting. I think I pretty much got it now. Sometimes I think education would have been useful, too late now.
All this time I thought it was the forth letter in the Greek alphabet.