A specific accent of the English language not native to any place.
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.