in: a dependent variable
is: a differential equation in which every term that contains at all contains no powers of and its derivatives other than the first, no functions of and its derivatives, and no products of and its derivatives among themselves. [M6.1, P5.5]
therefore is: of first degree. [M6.1]
is exemplified: by
(This is linear in 
but is also inhomogeneous unless 
)
has the property: if homogeneous, that a linear combination of solutions is also a solution.
Copyright 1997, The Open University