The range of frequencies that a communications medium can carry. For baseband networking media, the bandwidth also indicates the theoretical maximum amount of data that the medium can transfer. For broadband networking media, the bandwidth is measured by the variations that any single carrier frequency can carry, less the analog-to-digital conversion overhead.
Bandwidth is the data- transferring capacity of a system- how much information can be sent from one place to another in a given period of time. There are many ways to measure this- for example, the number of megabytes transferred per second.