The Collaborative International Dictionary of English v.0.48:

Biquadratic \Bi`quad*rat"ic\, 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]

WordNet (r) 3.0 (2006):

biquadratic equation
    n 1: an equation of the fourth degree [syn: biquadratic
         equation, biquadratic]