A function G: \{0, 1\}^\ell\to\{0,1\}^n with \ell \leq n is a pseudorandom generator against \mathcal A with bias \epsilon if, for every A in \mathcal A, the statistical distance between the distributions A(G(U_\ell)) and A(U_n) is at most \epsilon, where U_k is the uniform distribution on \{0,1\}^k.
The quantity \ell is called the seed length and the quantity n-\ell is called the stretch of the pseudorandom generator.