A receptive partner is someone who receives penetration, rather than giving it, during sex. The term most commonly refers to sexual roles during anal or oral sex. It can also be used to refer to the roles of gay men or BDSM couples.
The term receptive partner is often used interchangeably with bottom. However, receptive partner is a less ambiguous term as it makes clear that physical position isn't linked to a sexual role. Passive is another common synonym. This can also be ambiguous as a receptive partner takes the most active role in oral sex. Catcher is also a colloquial term for receptive partner.