Recall the Radon-Nikodym Theorem. The function $f$ that satisfies $$\nu(A)=\int\limits _{A}f \, d\mu$$is uniquely defined up to a μ-null set, that is, if $g$ is another function which satisfies the same property, then $f=g$ $\mu$-a.e.. The function $f$ is commonly written $\frac{d\nu}{d\mu}$ and is called the Radon-Nikodym derivative.