A plotter plans out their story (even if it changes on the run) and a pantser flies by the seat of the pants without a plan (even if they have to stop to do research or 'think about things for a while').
There are many writers who say they're pantsers (Mr King, for this example, is one of them), but when you know his history and education, you find that he's learned all the stuff about planning (things like structure, language, etc., studied poetry and literature) and probably learned it was easier to do it in his head. Once you know something well enough is the time to let go of the lessons and make a unique signature, perhaps?
Planning is more about finding the high and low points and progression markers to get to a satisfying end. If pantsers fly ahead until they get to the end and then have to go back to fill in all the points where a higher or lower point is needed and fix the beginning to determine what the question needs to be to ensure the reader understands how the end answers that question, it is only backward planning.
In the end, it's all about reader expectations, not how a writer gets a story on paper, and whether planner, pantser, or plantser is irrelevant at that point. In my opinion, anyway.