Bandwidth generally refers to the width of the frequency range over which a signal/system is valid.
For a signal, this is the width that a signal’s frequency spectrum is defined. A signal with a finite bandwidth is a bandlimited signal
For a system (i.e., a circuit), it’s the width of its frequency response over which it delivers its specified performance without distortion. Amplifier bandwidths are usually over 3 dB, and are designed such that the bandwidth coincides with a spectrum of signals intended for amplification.