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.,
Such equations are easy to solve, since they reduce to a quadratic equation in the variable and hence can be solved for using the quadratic formula
and then in terms of the original variables by taking the square
roots.