Overnight trading is trading that takes place outside the normal trading hours of the underlying exchange on which the asset is listed.
US stock brokers that allow overnight trading may extend their trading session after business hours until the opening of the next trading day.
The forex market stays basically open all week because trading is done by banks and businesses around the world. There is no formal overnight trading in the forex market because the market is always open.
Bonds have extended trading hours, and overnight trading in equities can occur from 4:00 AM to 9:30 AM ET (when the exchanges are open) until 4:00 PM ET. (when the exchanges close) and 20:00. ET.