1. Bandwidth is the data transfer rate of an electronic communications system. The bandwidth of a transmitted communications signal is a measure of the range of frequencies the signal occupies. In plain English bandwidth is a measurement of the running data from one computer to another. All transmitted signals, whether analog or digital, have a certain bandwidth. The same is true of receiving systems. An example it would take more bandwidth to up download a streaming video than to upload a html file. Bandwidth is directly proportional to the amount of data transmitted or received per unit time.
2. The carrying capacity or size of a communications channel; usually expressed in hertz (cycles per second) for analog circuits (the original meaning of the term) and in bits per second (bps) for digital circuits (the newer more applicable meaning).
3. Another term for capacity. The volume of data (usually measured in bits per seconds) that can be sent through the Internet connection (the modem or transmission line). Think of bandwidth as a road. The wider the road, the more traffic can get through. Although the Internet is huge, only a certain amount of data can pass through at a time.