I've always thought it means lightweight and below, or maybe everything below lightweight, but I don't know if I've ever seen it precisely defined. It's one of those boxing terms that gets interpreted in a lot of different ways, I think.
http://www.eastsideboxing.com/forum/showthread.php?t=215071 similar, but i guess it takes on what you're aiming for.
I think Barrera, Morales and Hamed pretty much legitimized the FW division as a solid class for the current generation, below, the bantams and flys and straws surely don't sound that same, wrongfully of course. For me, FW was the class years back I rated as "the last solid division" and "the rest below", which is not the case anymore.