Ghost is an achievement/trophy in Deus Ex: Mankind Divided, awarded for making your way through an area with hostiles, without being detected by any of them. This includes leaving no bodies for them to find, broken cameras that prompt 'searching'... You just simply ghosted it. A similar achievement with the same name is obtainable in Human Revolution.
When getting through a hostile area without being seen, you are awarded with the Ghost bonus. This is an XP bonus that is given any time you are not detected. The achievement itself is given the first time you get the Ghost bonus.