________> _______ aircraft A
...............X..............
________> __* _____ aircraft B
................0......Y...........
Aircraft A is abeam point X. Aircraft B is not yet abeam point Y, but it will be when it reaches the *. It is now abeam point O.
Technically his aircraft is always 90 degrees from the airport reference point, no matter what bearing the airport was in relation to his position.
...90 degrees to the right or left of the aircraft track .
Having drawn the picture, I have to agree that the definition is somewhat ambiguous. Speaking in terms of geometry, all those points are 90 degrees off the
track, because the "track" is a line, not a point. It would make better sense if they left "track" out of it (" 90 degrees to the right or left of the aircraft"), or made it even more wordy by saying something like "90 degrees to the right or left of the aircraft
with reference to its track".
I'm glad I don't have to write the glossary.