What is bandwidth?(communication and modulation)

Bandwidth is the difference between the upper and lower frequencies in a contiguous set of frequencies. It is typically measured in hertz, and may sometimes refer to passband bandwidth, sometimes to baseband bandwidth, depending on context.

A key characteristic of bandwidth is that a band of a given width can carry the same amount of information, regardless of where that band is located in the frequency spectrum (assuming equivalent noise level). For example, a 5 kHz band can carry a telephone conversation whether that band is at baseband (as in your POTS telephone line) or modulated to some higher (passband) frequency.