In the context of human sexual behaviour, especially anal sex among gay men, a bottom is a receptive partner (i.e. a partner who is penetrated), or a person who prefers the receptive role. The counterpart is a top.
"Bottom" can also be used as an intransitive verb meaning to be sexually penetrated by: to bottom for someone.
It is very commonplace for partners to change roles from one encounter to the next or to have both insertive and receptive sex, although many people have a preference between roles. A person who enjoys both topping and bottoming is called versatile or a switch.
The terms "submissive" or "passive" have been used for "bottom," though these may be confusing as the sex in question needn't be part of a dominance relationship, nor is the bottom necessarily any less "active" than the top.
By extension, the term "bottom" is also used in BDSM to mean a partner who receives stimulation from another, and who may or may not be submissive. See bottom (BDSM).
Bottoms sometimes identify themselves by wearing a set of keys on the right side of the belt or a colour-coded handkerchief in their right rear pocket. This practice, called flagging, began in the gay male subculture but appears to be dying out.
In sexually-themed anime and manga, especially shounen-ai and yaoi, a bottom is referred to as uke.