Anscombe's Quartet is a collection of four two-variable datasets that have the same mean, variance and correlation, but very different characteristics despite that. They illustrate the importance of looking at the data using various visualisations as well as calculating numerical summary statistics.
Used in glossary entries: correlation, mean (μ), summary statistics, variance
Figure 14.4 Anscombe's Quartet (image Schutz and R team, https://commons.wikimedia.org/w/index.php?curid=9838454)