A true “bucket seat” is designed like a racing seat to provide lateral support. One cannot move around and “fidget” in a good bucket seat. It holds you right where you’re supposed to be for the best vehicle control.
A “sport seat” is actually a seat designed to be a seperate seat in and of itself but to provide the person with the ability to do all the moving around an fidgeting that the average driver does on a long drive.
But the terms are very loosely used.