The term "biquadratic equation" is sometimes used to as a synonym for quartic equation (Beyer 1987b, p. 34), but perhaps more commonly (e.g., Hazewinkel 1988; Gellert et al. 1989, p. 101) and more properly for a quartic equation having no odd powers, i.e., z^4 + a_2 z^2 + a_0 = 0. Such equations are easy to solve, since they reduce to a quadratic equation in the variable x = z^2 and hence can be solved for x using the quadratic formula and then in terms of the original variables by taking the square roots.

quadratic equation | quartic equation