Biquadratic Bi`quad*rat"ic, a. [Pref. bi- + quadratic: cf. F. biquadratique.] (Math.) Of or pertaining to the biquadrate, or fourth power. Biquadratic equation (Alg.), an equation of the fourth degree, or an equation in some term of which the unknown quantity is raised to the fourth power. Biquadratic root of a number, the square root of the square root of that number. Thus the square root of 81 is 9, and the square root of 9 is 3, which is the biquadratic root of 81. Hutton.