The Collaborative International Dictionary of English v.0.48:

Biquadrate \Bi*quad"rate\, n. [Pref. bi- + quadrate.] (Math.)
   The fourth power, or the square of the square. Thus 4x4=16,
   the square of 4, and 16x16=256, the biquadrate of 4.
   [1913 Webster]

WordNet (r) 3.0 (2006):

biquadrate
    n 1: an algebraic equation of the fourth degree [syn:
         biquadrate, biquadratic, quartic, fourth power]