A frequency division multiplexed communications channel, ie one that uses different parts of the frequency spectrum for different signals.

Normal phone service uses approximately 300Hz-3.2KHz for voice signal, 75V at 20Hz for ringing, and a DC loop current to signal on/offhook and supply a small amount of power to subscriber equipment. However, a pair of copper wires can carry frequencies far above 4KHz. ADSL exploits this extra Analog BandWidth to carry analog-encoded data between the subscriber and telco without interfering with normal phone service.

You could even argue that POTS is already broadband since it uses loop current to signal when the line is in use..

The term has long become synonymous with fast, high-bandwidth networking, regardless of the means to achieve it. TelecomNZ uses it to describe 128k connections.

See also Dictionary:Broadband

Compare BaseBand