What are pass band Ripples in a Filter?

Ripples are the fluctuations (measured in dB) in the pass band, or stop band, of a filter’s frequency magnitude response curve. Elliptic and Chebyshev-based filters have constant ripple across their pass bands. While Bessel and Butterworth derived filters have no ripple in their pass band responses. Ripples in the stop band response are called as out-of-band ripple.