Bandwidth refers specifically to the dimension of the frequency spectrum used by a carrier. For example, analog voice typically uses about 4000 Hz of frequency spectrum, so the bandwidth of a call is 4000 Hz or 4 kHz.
More generally, in data networking, the term bandwidth is used as a synonym for throughput. This is because there is a specific mathematical relationship between bandwidth and possible throughput. This relationship is expressed by the Nyquist Theorem. According to this theorem, the maximum data sampling rate in bits is twice the available spectrum. Thus a 8000 bits per second data sample fills up a 4000 Hz channel.
The relationship between bandwidth and throughput is more complex than a simple application of this formula for the Nyquist rate. Still, in general, there is a significant relationship between the length of the band (frequency spectrum) and the possible speed of transmission over that band. For this reason, bandwidth is most commonly used to mean speed, data rate, or throughput.