I have one good argument against it. "Galactic Aphelion", seriously?
It's the area opposite from Sol in Galactic co-ordinates. That is the more general meaning of the Greek word "aphelion": the more specific meaning in orbital mechanics is derived from that. So it's a wholly appropriate name.
What's your problem?
No, aphelion is specifically the highest point in the orbit around the Sun and my five-minute google search shows no sources confirming your hypothesis. The same search indicates that aphelion translates literally as "away from the sun". And while I really dislike almost all of those names, this is the only one (as far as I can see) that uses a word improperly.
Aphelion is specifically the farthest a body gets from the sun
the generic term for the farthest separation between any two orbiting bodies being apogee.
There is no usage more improper than your use of an adjective implying elevation rather than just distance.
I'm afraid you're wrong, the generic term you're looking for is "apsis" or "apoapsis", "apogee" refers to the maximum distance the from Earth in an orbit around it.
Whoops, you are correct there, I did indeed type the wrong word. *hangs head*
You're wrong everywhere else though. Nobody is suggesting that Galactic Aphelion was a thing before it was used by the GMP, it was a term coined for the GMP to take the existing meaning of aphelion and use it to name the region of the galaxy farthest from Sol. It is an appropriate usage, and I really don't see why you have such an issue with it.
There's only one meaning of aphelion in English (all of the definitions I could find mention orbits), the one you refer to exists only in Greek. So it's not an improper usage of the word, but a piece of gratuitous Greek that masquerades as an improper usage of an existing astronomical term? That's hardly any better.
There's only one meaning of aphelion in English (all of the definitions I could find mention orbits), the one you refer to exists only in Greek. So it's not an improper usage of the word, but a piece of gratuitous Greek that masquerades as an improper usage of an existing astronomical term? That's hardly any better.
Edit: One more thing I've just realized - "aphelion" translates from Greek as "away from the Sun", not "the furthest away from the Sun", notice the lack of the superlative in the former. Now not only it doesn't make sense when using the English definition, the usage of Greek term is pointless too, as every single point in the universe that isn't the Sun is "away from the sun".