Silverlight is a city in the Cosmere, located in the Cognitive Realm. It is where Khriss, Nazh and Guyn reside, and is most likely the meeting point for Worldhoppers and Realmatically aware experts. Little else is known about Silverlight at this time.
Brandon was hesitant about namedropping this city because he thought he might have to change the name with the release of Microsoft Silverlight.
It has been postulated by a reader that Silverlight is that place that is not a world upon which a Shard is held, but by whom none yet know.