Curious... What does a mid-Atlantic accent sound like?
Mr. Burns: https://www.youtube.com/watch?v=othBTFk_W1Y
Sometimes it means "trans-Atlantic", an accent which Buckley, Burns, and many pre-1950 movie stars spoke with (while on-screen anyway). It falls somewhere between the accents found in the USA and accents found in the UK.
In the context of this thread, I think it means the mid-Atlantic states in the US. It's close to the American "television news accent".
EDITED TO ADD: It's worth noting that the two accents called "mid-Atlantic" sound very different.
That said, people today often use "mid Atlantic accent" to mean the standard "region-less" accent used by, for example, TV announcers. It is more and more the standard accent of America's educated elite in urban centers, regardless of location.