Also, as a slang term, anyone (but most used to describe a gay man) in a passive or receptive role during sexual activity. In heterosexual sexual activity the role of who enters who is, unless there are harnesses and toys involved, dictated by gender and obvious. Not so much in the case when one deviates from that norm.
It's not a pejorative slang, but merely a descriptor. In a personals ad, someone describing himself as a bottom wishes to be receptive if the sex is anal, or the one fellating if the sex is oral.
It's interesting to note that the notion of being receptive doesn't necessarily mean passive, however. There is the term "power bottom" which implies that the person may be receiving the generative member of the other person, but in no way in a passive manner, and in fact is more sexually aggressive and dominant in that role than the person "topping" the aforementioned bottom.
There's also the built-in pun factor considering that sexual activity in gay men sometimes involves the anus, buttocks and/or inner thighs, all of which are euphemistically, as noted above, referred to as a "bottom".