Let $(X,\Sigma)$ be a measurable space with measures $\mu$ and $\nu$ such that $\nu$ is absolutely continuous with respect to $\mu$. The Radon-Nikodym derivative $\frac{\text d\nu}{\text d\mu}$ is the function $f \in$ L$_1(\mu)$ such that $\nu(A) = \int_A f\text d\mu$.
Wikidata ID: Q1191319