Order of Differential Equations

The order of a differential equation is the highest order of the derivative appearing in the equation. Consider the following differential equations,

dy/dx = e^x, (d⁴y/dx⁴) + y = 0, (d³y/dx³) + x²(d²y/dx²) = 0

In above differential equation examples, the highest derivative are of first, fourth and third order respectively.