The USCG has a very specific definition for it: the longitudinal distance from the forward most part of the stem to the after most part of the stern, excluding things which are not part of the molded hull, i.e. bow pulpits, swim platforms, etc.
Do you know where that's documented? I've always wondered where that definition came from. I've heard it before, but it doesn't match how the term is used just about everywhere else.