Biquad is a second order filter with transfer function $H(z)=\dfrac{b_0 + b_1z^{-1} + b_2z^{-2}}{1+a_1z^{-1}+a_2z^{-2}}$ for digital systems and $H(s)=\dfrac{b_2s^2 + b_1s + b_0 }{s^2+\frac{\omega}{Q}s+\omega ^2}$ for analog systems.