Order of the Differential Equation
The order of the highest ordered derivative involved in a differential equation is called the order of the differential equation.
It may also be defined as: the order of a differential equation is the degree of the highest order derivative that occurs in the equation.
Examples:

The order of the differential equation $$\frac{{dy}}{{dx}} = y{e^x}$$ is one.

The order of the differential equation $$\frac{{{d^2}y}}{{d{x^2}}} + y = \sin x$$ is two.