Bandwidth is a measure of frequency range and is typically measured in hertz. Bandwidth is a central concept in many fields, including information theory, radio communications, signal processing, and spectroscopy. Bandwidth also refers to data rates when communicating over certain media or devices.
In a digital communication system, bandwidth has a dual meaning. In the technical sense, it is slang for baud, the rate at which symbols may be transmitted through the system. It is also used in the colloquial sense to describe channel capacity, the rate at which bits may be transmitted through the system (see Shannon Limit). Hence, a 66 MHz digital data bus with 32 separate data lines may properly be said to have a bandwidth of 66 MHz and a capacity of 2.1 Gbit/s — but it would not be surprising to hear such a bus described as having a "bandwidth of 2.1 Gbit/s." Similar confusion exists for analog modems, where each symbol carries multiple bits of information so that a modem may transmit 56 kbit/s of information over a phone line with a bandwidth of only 12 kHz. A related metric which is used to measure the aggregated bandwidth of a whole network is bisection bandwidth.
Bandwidth is also used in the sense of commodity, referring to something limited or something costing money. Thus, communication costs bandwidth, and improper use of someone else's bandwidth may be called bandwidth theft.
Meaning of bandwidth in web hosting
In website hosting, bandwidth is the amount of information downloadable from the webserver over a prescribed period of time. In essence, it is the rate [data/time], but the time in this case is not seconds but rather a month or a week. So this rate is not like 56K or broadband, etc., which are also bandwidth but are measured per second. Web hosting companies often quote a monthly bandwidth limit for a website, for example 100 GB/month. If visitors to the website download a total greater than 100 GB in one month, the bandwidth limit will have been exceeded.