Boondocking is essentially the same as what the Europeans call wild camping.
The boondocks are the back of nowhere.
Dry camping means camping without hookups. No shore power, no water hookup or sewer connection. You can dry camp on a city street, in a Walmarts parking lot, or out in the Boondocks.
Stealth camping means you don't want to call attention to yourself. Parking on a city street is usually stealth camping, parking at a Walmarts isn't. Boondocking might be if you go to some trouble to get far enough into the back country that you are unlikely to be seen by anyone.
Btw, during WW2, the field shoes worn by American Marines were known as boondockers. The story is that the name came from some undeveloped island in the Phillipines where the Marines were stationed during the Phillipine Insurrection.
Boondock is the Americanization of the native name.
Regards
John