\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.

\Bi`quad*rat"ic\, n. (Math.)
(a) A biquadrate.
(b) A biquadratic equation.