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.
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