Someone who is penetrated during sex, not to be confused with a sub who is the one receiving the power. Bottom and top are terms purely defined by penetration.
Even when Phil is being amazon pressed by his girlfriend he is still not a bottom, but he is submissive.
Monty walks in a traincart and thinks "this isn't the best place to fuck someone but since I'm a slutty bottom this is the best place to bottom for someone"