Let \text{Ext}: \{0,1\}^n \times \{0,1\}^d \to \{0,1\}^m be a function that takes as input a sample from an (n, k) distribution X and a d-bit seed from U_d, and outputs an m-bit string.
\text{Ext} is a (k, ε)-extractor, if for all (n, k) distributions X, the output distribution of \text{Ext} is ε-close to U_m.