 We are grateful to Liam Kofi Bright and a very gracious anonymous reviewer for their comments and suggestions. We are also grateful to Branden Fitelson, Alan Hájek, and Wolfgang Spohn for their feedback on the previous version of this entry. We have used material from Huber (2009) and Genin (2019). Supported by the German Research Foundation through the Cluster of Excellence “Machine Learning – New Perspectives for Science”, EXC 2064/1, project number 390727645.