Name collisions

Hey everyone, I coded a little platforming game and a seperate level editor. Now I want to combine them to a single game but I got a little problem. The classes of the game itself have the same names as the classes of the level editor (e.g. Player, Item, Box,...). I didn't merge the two games because obviously I would get a lot of errors because the classes are not unique. Can you recommend me a way of preventing this? Is there maybe an option to define namespaces and use the classes like ? Thank you for your help.