an algebraic equation or function of the fourth degree
an equation of the fourth degree
a polynomial of the fourth degree
A biquadratic equation.
involving the fourth and no higher power of a quantity or degree
Of a polynomial expression, involving only the second, third and fourth powers of a variable, as x⁴ + 3x² + 2. Sometimes extended to any expression involving the biquadrate or fourth power (but no higher powers), as x⁴ − 4x³ + 3x² − x + 1.