**Definition** An [[Functional Equation|functional equation]] containing the [[Fréchet Differentiation|derivatives]] of one or more dependent variables with respect to one or more independent variables, is said to be a *differential equation (DE)*. See [[Integral Equation]]. # Properties We can classify a DE by: 1. Number of dependent variables. If a DE has only one then it is an [[Ordinary Differential Equation]] otherwise it is a [[Partial differential equations]]. 2. [[Order of a Differential Equation]].