Pseudorandom ensemble

In cryptography, a pseudorandom ensemble is a family of variables meeting the following criteria:

Let ${\displaystyle U=\{U_{n}\}_{n\in \mathbb {N} }}$ be a uniform ensemble and ${\displaystyle X=\{X_{n}\}_{n\in \mathbb {N} }}$ be an ensemble. The ensemble ${\displaystyle X}$ is called pseudorandom if ${\displaystyle X}$ and ${\displaystyle U}$ are indistinguishable in polynomial time.