The term bandwidth refers to the width of a band, the difference between to frequencies. (In portions of the spectrum where wavelength is the typical unit used, it may be characterized as the distance between two wavelengths.) The term is used in astronomy, communications, and signal processing. It can be the characteristic of some electronics (an amplifier) or a transmission line, and is used as such in radio astronomy. It can also be deliberately imposed, e.g., by a filter. If a filter (or electronics) chops off the frequencies at a point, the difference is easily calculated, but typically the sensitivity decreases over a portion of the spectrum, and one method of characterizing the bandwidth is full width at half maximum.
To make up an example, a sensor aiming to sense 100MHz may actually sense from 90MHz to 110MHz, giving it a bandwidth of 20MHz.
Broadband and narrowband are commonly used terms to characterize a bandwidth as large or small.