Szemerédi regularity lemma
In extremal graph theory, Szemerédi's regularity lemma states that a graph can be partitioned into a bounded number of parts so that the edges between parts are regular (in the sense defined below). The lemma shows that certain properties of random graphs can be applied to dense graphs like counting the copies of a given subgraph within graphs.
Source: Wikipedia — Szemerédi regularity lemma (CC BY-SA 4.0)