A specific accent of the English language not native to any place.I also found a lot of references to "transatlantic accent" where the writer may or may not have known what they were talking about. A lot of people seem to think that a transatlantic accent is just what someone from England sounds like when he's trying to suppress his English accent. There are also references to "fake" transatlantic accents, which makes no sense if the accent is artificial.
It is about halfway between British and American. It is considered an advantage for an actor to be able to speak transatlantic properly (it is not just a random mix of British and American but a very specific accent having its own exact rules).
It is often considered hard to master because if not done absolutely right it sounds completely fake and ridiculous. When done right, it sounds very sophisticated.
The transatlantic accent is generally used for characters that cannot speak with any real English accent, for example aliens in science fiction, or non-English characters in historical fiction. Also used by God to express the idea that God does not belong to any particular place or culture but transcends them all.
The transatlantic accent is often recommended to immigrants from non-English speaking countries. This is because people are used to this accent from movies, so, ironically, it does not sound foreign when spoken by immigrants.
What I was hoping to find was some fun history such as that it was invented by an early 20th-century linguist under contract from the first radio network or that it was taught at academies for young ladies of a certain social class or some such. But so far, nothing interesting has come up.