What is bandwidth in the context of electrical circuits?

Bandwidth is the range of frequencies over which a circuit operates effectively, typically defined as the frequency range where output power is at least half of maximum (3 dB points). For resonant circuits, BW = fr/Q, so higher Q means narrower bandwidth. In amplifiers, bandwidth indicates the useful frequency range. The gain-bandwidth product is constant for many amplifiers, creating a tradeoff between gain and frequency response.