The portion of spectrum, between limiting frequencies (or, in the optical regime, limiting wavelengths,) that is transmitted with minimum relative loss or maximum relative gain. Note 1: The limiting frequencies are defined as those at which the relative intensity or power decreases to a specified fraction of the maximum intensity or power. This decrease in power is often specified to be the half-power points, i.e., 3 dB below the maximum power. Note 2: The difference between the limiting frequencies is called the bandwidth, and is expressed in hertz (in the optical regime, in nanometers or micrometers. )