Averages of $b$-hadron, $c$-hadron, and $\tau$-lepton properties as of 2021

This paper reports world averages of measurements of <math display="inline"><mi>b</mi></math>-hadron, <math display="inline"><mi>c</mi></math>-hadron, and <math display="inline"><mi>τ</mi></math>-lepton properties obtained by the Heavy Flavor Averaging Group using results available before April 2021. In rare cases, significant results obtained several months later are also used. For the averaging, common input parameters used in the various analyses are adjusted (rescaled) to common values, and known correlations are taken into account. The averages include branching fractions, lifetimes, neutral meson mixing parameters, <math display="inline"><mi>C</mi><mi>P</mi></math> violation parameters, parameters of semileptonic decays, and Cabibbo-Kobayashi-Maskawa matrix elements.