What is dongle???

Hi.
I have installed AvidExpress Pro.
now when I double click on its Icon. the error message box is shows this error..

No dongle was found. Please connect a valid dongle
to this computer to ensure proper operation.

anyone can tell me what is this???

The term "dongle" is used to describe a very short cable.

In this case, the dongle is likely to be a hardware "key" for the software you're trying to use. The requirement for a such a key is a prevention against the unauthorized duplication or installation of the related software program. Without the correct dongle connected (usually to a serial port), the software will not run.