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 amazonpressed by his girlfriend he is still not a bottom, but he is submissive.
Monty walks in a train cart 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"