While there certainly is a book definition somewhere, one size does not fit all. Half/Full Duplex only uses one channel so that anolgy falls apart some. As an example I signed onto Fidonet in 1984 using a 300 Baud Full duplex modem. Wouldn't call that Broadband would ya?
I think the full definition of Broadband should be that you have two channels, able to send and receive simultaneously data/voice/video/whatever at a specific rate. The issue we're having here is what is that Rate Number? The FCC calls it 200kbps. ISDN BRI is 128kbps. Ergo, not Broadband.
When the shop I was working for installed ISDN BRI in 1996, we thought it was Broadband, by today's standards, it's not. In fact, as we move forward, the number is going to get higher. As for just surfing the Internet, right now, a 256kbps connection is just barely enough for my Definition. Others push the number much higher. It's a fluid definition because content gets richer, mpgs get bigger, and what works today, might not be all that useful 10 years from now. Right now, far as I know, only the FCC has defined the speed as 200kbps. When some other authoratative body weighs in all calls a number we might get a clearer definition.