Synonyms for Biquadratic (same or very similar meaning)
WordNet sense 1 (an algebraic equation of the fourth degree):
biquadrate, quartic, fourth powerWordNet sense 2 (an equation of the fourth degree):
biquadratic equationWordNet sense 3 (a polynomial of the fourth degree):
biquadratic polynomial, quartic polynomialFrom the ODE community, based on WordNetadd/edit