Y'+p(x)y+q(x)=0 is called "linearity".
For example:
Y'=sin(x)y is linear.
But y' = y 2 is not linear.
Extended data
The so-called linear differential equation, in which:
A, only the function itself and the derivative function of any order of the function can appear;
B, there can be no operations other than addition and subtraction between the function itself and all derivative functions;
C, the function itself and itself, each derivative function itself and itself, cannot have any operation except addition and subtraction;
D, it is not allowed to do any form of compound operation on the function itself and the derivative functions of each order.