Frequency bandwidth which transfers 95% of power of the signal
You integrate the PSD (power spectral density) of the signal over various widths centered on the center of the channel until you get 95% of the total.

This can be done analytically for some signals, but must be determined empirically for others.

It's an interesting experiment to take the FFT of a signal to get it's PSD, truncate that spectrum to various bandwidths, and then perform the inverse FFT to see how the original waveform gets distorted &mdash; and for digital signals, what happens to the eye diagram. It can give you some real insight into what happens with real-world communications channels.