The frequency at which a highpass or lowpass filter's response curve has dropped 3dB below the passband level. This is an electrical engineering convention. The term is somewhat misleading since the cutoff frequency is not the point at which the filter completely blocks frequencies, but rather the point at which the filter has just begun to take effect. 3 dB isn't very much (it's usually considered the "just noticable difference"), and if a highpass or lowpass filter is to be used to block a specific frequency or range of frequencies, it is necessary to offset the cutoff frequency somewhat from the range of frequencies to be blocked, depending on the filter's slope. If the filter employs resonance, then the cutoff frequency will usually actually correspond to a peak in the response curve, with the actual cutoff beginning on the other side of the peak.