Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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