A person who prefers to be penetrated, rather than doing the penetration.
The opposite of a Top, in which the person receives the penetration from another. In many instances, there is objection to the term 'bottom' as that it implies a lesser role, or some other political correctness nonsense, so generally a bottom is now called 'passive'.
Again, being called passive does not imply a lack of activity, simply a term for being politically correct.
Note - in BDSM playing, a submissive does not mean a bottom, in the strictest sense. Nor does passive refer to a lack of activity. Mainly it means one who performs under command.
This is where a person plays the role or is the one who gives into providing the pleasures of the other, mainly considered to be the one who is penetrated.
However, it can be where one submits to the other sexual needs of their partner, such as in BDSM sessions. This would then be the person receiving the various actions, such as being whipped, restrained, or following the other wishes of their partner.
Noteworthy: In BDSM, a bottom does not mean being the submissive. It can be the 'power' player, but who prefers being penetrated. In addition, they may prefer the authority it gives them, in commanding the penetration at their will, part of being the dominant player, or power player.