Dielectric absorption is the name given to the effect by which a capacitor that has been charged for a long time discharges only incompletely when briefly discharged. Although an ideal capacitor would remain at zero volts after being discharged, real capacitors will develop a small voltage, a phenomenon that is also called soakage or battery action. For some dielectrics, such as many polymer films, the resulting voltage may be less than 1-2% of the original voltage, but it can be as much as 15 - 25% for electrolytic capacitors or supercapacitors. In order to prevent shocks most very large capacitors are shipped with shorting wires that need to be removed before they are used.
See permittivity and Types of capacitor#Dielectric_absorption (soakage).