While the resonant frequency of a tuned circuit is a single frequency, the effect of resonance is significant over a certain range of frequencies. What is this range called?

The bandwidth of a tuned circuit is the specific range of frequencies over which the circuit's response remains strong or significant. In engineering terms, this is typically measured between the upper and lower frequencies where the signal power drops to half of its peak value (the -3 dB points). While resonance mathematically occurs at one exact frequency, practical circuits do not completely block or pass frequencies immediately adjacent to that point. Instead, they exhibit a bell-like curve of decreasing effectiveness, and the usable width of this response curve is the bandwidth. Although "Quality factor" (Q) is closely related because a higher Q results in a narrower bandwidth, the Q factor is a dimensionless ratio of energy stored to energy lost, not the frequency range itself.