Bandwidth is the difference between the upper and lower cutoff frequencies of for example a filter, a communication channel or a signal spectrum, and is typically measured in hertz. Bandwidth in Hertz is a central concept in many fields, including electronics, information theory, radio communications, signal processing, and spectroscopy.
In computer networking literature, digital bandwidth often refers to data rate measured in bit/s, for example channel capacity (digital bandwidth capacity) or throughput (digital bandwidth consumption). The reason for this usage is that the channel capacity in bit/s is proportional to the analogue bandwidth in Hertz according to Hartley's law.