Ghost (Mario Kart series)
From the Super Mario Wiki
A Ghost is the recording of a player's driving performance in the Mario Kart series. The concept of Ghosts has appeared in most games in the series. Ghost drivers appear as semi-transparent characters and behave as players who used them did while achieving a high score. These drivers cannot be touched and pass through karts and obstacles. Driving against one's own Ghost enables a player to directly compete against their own record. Ghost data can only be recorded in Time Trial mode; recording this data may be impossible in races that are too long in some games due to memory limitations.
Starting with Mario Kart: Double Dash!!, there are Staff Ghosts, which are made by Nintendo as the main course ghosts. In Mario Kart Wii and Mario Kart 7, there are Normal and Expert Staff Ghosts. Throughout the games, some tracks have reused characters, such as Mario for Mario Circuit, Bowser for Bowser's Castle, etc.
In Mario Kart 7, players who are met online will be added as Ghost Data.
In Mario Kart 8 in team battle mode, if the player loses all of their balloons, they will play as a ghost, but they won't be able to get any points.