In general, the difference between the highest and lowest frequencies in a given range of frequencies for an analog signal. For example, if the lowest and highest frequencies a telephone line can carry are 300 Hz and 3300 Hz, the telephone line can accommodate a bandwidth of 3300 - 300 = 3000 Hz, or 3 kHz.
In computer networking with digital signals, bandwidth is the capacity of a communication channel for carrying signals. The greater the bandwidth, the more data can be transferred in a given time. Bandwidth is sometimes referred to as “throughput,” and for digital communication, it is usually measured in bits per second (bps) or a multiple thereof (Kbps, Mbps, Gbps, and so on).
For fiber-optic cabling, the bandwidth is usually expressed in units of MHz-km. For example, a cable rated at 500 MHz-km could carry 500 Mbps of data a distance of 1 kilometer, 250 Mbps of data a distance of 2 kilometers, 100 Mbps of data a distance of 5 kilometers, and so on.