I was helping a friend set up a simple home network after a move recently. He referred to his ONT as a modem, and I was tempted to say "well, actually, that's not a modem..."
I didn't, because I realize that from a non-technical perspective, "modem" effectively means "the box from my ISP that I plug the wireless router into".
But from a technical perspective, is an ONT appropriately described as a (type of) modem? I'm not a big hardware person, but my instinct is no, because there's no carrier wave with fiber. The modem article on wikipedia has a section on optical modems that talks about optical network units... so, bonus question: are ONU and ONT synonymous?