Source : Webster's Revised Unabridged Dictionary (1913)

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.

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

Source : WordNet®

biquadratic
     adj : of or relating to the fourth power

biquadratic
     n 1: an algebraic equation of the fourth degree [syn: {biquadrate},
           {quartic}, {fourth power}]
     2: an equation of the fourth degree [syn: {biquadratic equation}]
     3: a polynomial of the fourth degree [syn: {biquadratic
        polynomial}, {quartic polynomial}]