is a capacitor deployed at the supply rails with the main function of filtering AC transitions on a DC signal.
Bypass capacitors are deployed at the supply rails with the main function of filtering AC transitions on a DC signal, in order to get a pure DC signal on the input.
Further reading
... or watching
EEVblog #859 - Bypass Capacitor Tutorial
w2aew #257: Power Supply Decoupling & Filtering: why we use multiple caps in different locations (experiment video)