 Permutation entropy is a measure of complexity used to analyze time series data. It can be calculated from any time series, regardless of its complexity or structure, and provides a simple way to quantify the amount of information present in the data. This makes it useful for analyzing economic markets and biological systems, where other measures are too complicated or expensive to use. This article was authored by Osvaldo Arasso, David Papo, Luciano Zunino, and others.