computationally indistinguishable

If ${\{D_n\}}_{n\in \mathbb{N}}$ and ${\{E_n\}}_{n\in \mathbb{N}}$ are distribution ensembles (on $\mathrm{\Omega }$) then we say they are computationally indistinguishable if for any probabilistic, polynomial time algorithm $A$ and any polynomal function $f$ there is some $m$ such that for all $n>m$:

where ${\mathrm{Prob}}_A(D_n)$ is the probability that $A$ accepts $x$ where $x$ is chosen according to the distribution $D_n$.