A probability kernel from (Y,T)(Y,\mathcal T) to (X,Σ)(X,\Sigma) assigns a μy\mu_y on XX to each y∈Yy\in Y, such that y↦μy(E)y\mapsto\mu_y(E) is measurable for every E∈ΣE\in\Sigma.

Integrating a kernel

Given probability ν\nu on YY, the formula μ(E)=∫Yμy(E) dν(y)\mu(E)=\int_Y\mu_y(E)\,d\nu(y) defines a probability measure on XX. A uses such a kernel to describe conditional probabilities on fibers of a measurable map.