Silverlight Runtime is a browser plug-in to supportSilverlight enabled applications. If the Silverlight runtime is not installed,browsers will not be able to run Silverlight elements in the browser. You canset up your Silverlight tags in such a way that your browser will automaticallyprompt the user to download and install the Silverlight plug-in when yourapplication is launched in the browser.
Installing the run time is a onetime operation on theclient. Once installed, it will be automatically launched when any Silverlightapplication is loaded in the browser.
Note: Silverlight plug-in and the runtime, both are thesame; however, I am giving two definitions here.