Definition of cut-off frequencies and bandwidth:
The range of frequencies can be specified over which the gain does not deviate more than 70.7% of the maximum gain at some reference mid-frequency.
From above figure, the frequencies f1& f2 are called lower cut-off and upper cut-off frequencies.
Bandwidth of the amplifier is defined as the difference between f2& f1. Bandwidth of the amplifier = f2 - f1
The frequency f2 lies in high frequency region while frequency f1 lies in low frequency region. These two frequencies are also called as half-power frequencies since gain or output voltage drops to 70.7% of maximum value and this represents a power level of one half the power at the reference frequency in mid-frequency region.