The range of frequencies which a filter allows to pass through. For example, in a low pass filter, the passband consists of the frequencies below the cutoff frequency (hence the name). An ideal filter has an absolutely flat response in the passband. But real filters, and particularly filters used in synths, often have small deviations in the passband, places where the response varies by a few dB up or down. These may become more pronounced as resonance is increased, which is one of the factors that gives certain filter designs a characteristic sound.