*biquadratic*

a. [ Pref. bi- + quadratic: cf. F. biquadratique. ] (Math.) Of or pertaining to the biquadrate, or fourth power. [ 1913 Webster ]

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. [ 1913 Webster ]