Differential Equations Definition

Q. Which of the following are differential equations ?

1. $\frac{dz}{dy}=z$
2. $\frac{dz}{dy}\times z=0$
3. $x^2+y^2+2xy+2x-3y+25=0$
4. $w^2+d^2+\frac{dw}{dx}+5=0$