Pseudorandom ensemble

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

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