An {\displaystyle } -extractor is a bipartite graph with N {\displaystyle N} nodes on the left and M {\displaystyle M} nodes on the right such that each node on the left has D {\displaystyle D} neighbors, which has the added property that for any subset A {\displaystyle A} of the left vertices of size at least K {\displaystyle K}, the distribution on right vertices obtained by choosing a random node in A {\displaystyle A} and then following a random edge to get a node x on the right side is ϵ {\displaystyle \epsilon } -close to the uniform distribution in terms of total variation distance. Wikipedia