It is the range of frequencies occupied by a radio signal.
A small bandwith can yield a great reception sensitivity and
a part of electromagnetic spectrum can be used by more stations
at the same time.
Hams try to use modulation modes minimizing the required bandwidth,
such as CW and
SSB.
What follows is the typical bandwidth for each mode, assuming
audio from 300 Hz to 3 kHz, which is standard in phone modes:
