Honestly, that’s a slang term, not something in sexology or sexual health. How (and if, I personally haaaate it when used to describe consensual sex, for so many reasons) people use it varies.
But when most people use it, they simply mean body parts where there’s an orifice and something that can be inside that orifice are interlocking in some way.
Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has. - Margaret Mead