Page "Bandwidth (signal processing)" Paragraph 11

The 3 dB bandwidth of an electronic filter or communication channel is the part of the system's frequency response that lies within 3 dB of the response at its peak, which in the passband filter case is typically at or near its center frequency, and in the lowpass filter is near 0 hertz.

If the maximum gain is 0 dB, the 3 dB gain is the range where the gain is more than-3dB, or the attenuation is less than + 3dB.

This is also the range of frequencies where the amplitude gain is above 70. 7 % of the maximum amplitude gain, and above half the maximum power gain.

This same " half power gain " convention is also used in spectral width, and more generally for extent of functions as full width at half maximum ( FWHM ).